Black Stories ~ Touhou Edition - Case 20: Down to the bottom of Misty Lake!

[Dead Princess Sakana]

the man shot himself because he saw, or thought he saw the ghost of a person he killed?

NO

Was the man in a church/morgue?

NO

did he shoot a mirror?

NO

Was the man aware that he would die upon committing suicide?

YES. He commited the suicide with the exact goal of dieing.

The man thought he had killed the person outside, and killed himself over it.

YES. He did not only think so, he really had a connection to the death, as was answered before.

However, needless to say, this alone does not suffice. What I want you to tell me is, what happened that lead to the death and  how is the man connected to it.

Is the man a criminal?

NO

was the man inside the victim's house?

NO

Was the man of a sane mind?

YES. He was fully aware of what had happened and how he was connected to it.

Did he have a significant loss that day?

NO, he did not lose anything significant to him, as in: a person, money or an object

Is this surprisingly hard?

Ohohoho. I can understand that you find this riddle to be quite hard. However I shall assure you that you, my dear players, appear to be marching into the exactly right direction in which the truth lies. I am confident that it will not take too long for you to solve this riddle.
I have solved many of those riddles with friends, and the number of questions that was asked here up to now is still below the number that is in average needed to solve a case.
I admit this specific one here may be hard, but it is a real classic one as well, and its hardness stems mostly in the unusual situation it depicts. In this riddle here, one specific detail will solve almost all of the case, but one has to ask for that detail first.
Let me tell you, there are riddles that I consider far harder than this one that I have available, but those are often also more interesting.

I might add another hint as to how you should ask your questions:
At this point, I assume everyone of you has an image in their mind of what you know has happened. Now take the assumptions you have made in this image, and check them once more. Was every assumption you made, and may it be a really small detail, confirmed by my humble self? If not, then ask it, for having your own assumptions confirmed or denied is vital for solving a case

[Hououin Kyouma]

Was he in a house?

was the man's job something related to fighting? (as in like in the FBI, police, military, etc

[Fightest]

I find the most challenging part to be working with strictly yes/no questions, but it's that restriction that makes it fun.

A comment:

The man was not at home, yet knew where to get a gun intimately enough to not even consider another alternative, which suggests he either carries a gun, or operates for a lengthy amount of time somewhere where he could easily do so.

Is the man any of the following: policeman, soldier, gun store salesman, hunter?

[Hououin Kyouma]

Also did he mistake the person outside for someone else?

[Dead Princess Sakana]

Was he in a house?

Certainly not the kind of house you are thinking of, so NO. He was in a building that might be called a house though.

was the man's job something related to fighting? (as in like in the FBI, police, military, etc

NO

I find the most challenging part to be working with strictly yes/no questions, but it's that restriction that makes it fun.

A comment:

The man was not at home, yet knew where to get a gun intimately enough to not even consider another alternative, which suggests he either carries a gun, or operates for a lengthy amount of time somewhere where he could easily do so.

Is the man any of the following: policeman, soldier, gun store salesman, hunter?

I see you have realized what makes this game special, and I am glad to see you like the challenge. As you can see I often use a little bit more than just 'YES/NO' while trying not to give too much information, because being too strict in the answers will make the game unnecessarily hard and frustrating for the players.

As for your question: NO
I shall say that the manner in which the man commited suicide is irrelevant, only the fact that he did is of importance.
I originally heard this riddle with the suicide by shooting, but the man might as well have slit his wrists, jumped down somewhere or strangled himself.
Thinking about it now, the gun is probably the method that makes least sense, but people in those riddles often seem to have a gun available for the sole purpose of shooting themselves.

Also did he mistake the person outside for someone else?

NO

[Pesco]

Would this man have died due to a direct cause with what happened outside if he did not commit suicide?

[Hououin Kyouma]

did the person outside commit suicide as well?

was it daytime?

was the man (inside the building) scared of death?

was he scared of blood?

[Dead Princess Sakana]

Would this man have died due to a direct cause with what happened outside if he did not commit suicide?

NO

did the person outside commit suicide as well?

NO

was it daytime?

YES. More precisely, the suicide happened in the early morning

was the man scared of death?

was he scared of blood?

Both are UNKNOWN. You may assume he was not scared more or less of any of these things than any other human.

[Hououin Kyouma]

Was this set in a urban area?

is the window open?

is the window broken?

[Dead Princess Sakana]

Was this set in a urban area?

NO

is the window open?

is the window broken?

Both are UNKNOWN. You may assume that the window was intact and closed all the time.

[Fightest]

Hmm, not really got anything insightful, so I'll make a few of my sillier theories:

The Resident Evil scenario:

The man is a worker in a sealed laboratory working with dangerous pathogens or the like. Upon seeing someone die outside the sealed area through an observation window, he realised that he caused a containment breach. Realising that there was no longer anywhere he could go, he committed suicide to avoid death by starvation etc., also shock.

[Dead Princess Sakana]

The man is a worker in a sealed laboratory working with dangerous pathogens or the like. Upon seeing someone die outside the sealed area through an observation window, he realised that he caused a containment breach. Realising that there was no longer anywhere he could go, he committed suicide to avoid death by starvation etc., also shock.

Well, I think you already expect that, but NO. The man is not a scientist or anything alike and therefore did not cause any virus outbreaks or contaminations.

However, I may extract the following from your theory:
The man is at his workplace.
The death outside is somehow connected to his work.
Those I shall answer with YES.

[Fightest]

Was the death of the person outside caused by the man's negligence?

If not, was it caused by malicious intent?

[Dead Princess Sakana]

Was the death of the person outside caused by the man's negligence?

YES
Therefore:

If not, was it caused by malicious intent?

NO

[Fightest]

Was it the man's job to make sure such a death wouldn't happen?

[Thaws]

The man saw a person died due to him neglecting to do a certain procedure in his work and suicide out of guilt. Is this statement correct?

Was the death of the person outside caused by physical harm?

Was the death of the person outside caused by lack of air?

Was the death of the person outside caused by a human? (As in things like being stabbed/shot/etc)

[Spidere]

Is the man a construction worker?

If not, is he a sailor?

As soon as we understand the mans job, we have this.

[Dead Princess Sakana]

Was it the man's job to make sure such a death wouldn't happen?

YES

The man saw a person died due to him neglecting to do a certain procedure in his work and suicide out of guilt. Is this statement correct?

YES

Was the death of the person outside caused by physical harm?

YES

Was the death of the person outside caused by lack of air?

YES

Either of the above two is a possibility and will lead to the same result

Was the death of the person outside caused by a human?

NO

Is the man a construction worker?

NO

If not, is he a sailor?

NO

As soon as we understand the mans job, we have this.

Indeed, you are well on your path to the truth.

[Rin Kagamine]

Is the house a Greenhouse?

Was the man a gardener?

[Thaws]

Is how the man outside died relevant?

Is the man a security guard?

Did all this happen in somewhere where there's water(I mean places near lakes, seas, swimming pools, etc)?

[Hououin Kyouma]

was the person who died a customer?

[Fightest]

Is the house a Greenhouse?

What?

[Rin Kagamine]

What?

Really?

[Pesco]

Is the man a construction worker?

If not, is he a sailor?

You mean: Is this man one of the Village People? :V

[FinnKaenbyou]

Was he a terrorist who tried to send a parcel bomb and accidentally blew up the mailman?

*prepares for awkward glances*

[Hououin Kyouma]

originally the man had a plan to kill someone else by using some kind of mechanism but he accidentally killed someone who wasn't involved and because of that he felt guilty so he killed himself.

if not, does his job require a gun

[Fightest]

Really?

My surprise was at how a greenhouse could be related to a person dying from asphyxiation.

[Dead Princess Sakana]

Is the house a Greenhouse?

NO

Was the man a gardener?

NO.

Is how the man outside died relevant?

YES, as far as the general circumstances surrounding the death are concerned. The specific cause of death is irrelevant, as was stated before.

What you just did here, Sir Thaws, is a very clever move indeed. It is allowed for the players to ask for the relevance of something in general before they lose themselves in asking for all varieties of something.

Is the man a security guard?

NO

Did all this happen in somewhere where there's water(I mean places near lakes, seas, swimming pools, etc)?

YES

You mean: Is this man one of the Village People? :V

To my knowledge, he was not, though I can not say whether he did not like to dress in a similar fashion and sing YMCA occasionally. The thought scares me, however.

Was he a terrorist who tried to send a parcel bomb and accidentally blew up the mailman?

NO, though I would almost like to accept this solution, as it is wonderfully creative.

originally the man had a plan to kill someone else by using some kind of mechanism but he accidentally killed someone who wasn't involved and because of that he felt guilty so he killed himself.

NO

if not, does his job require a gun

NO, in fact there is no relation between his job and any kind of fireams or other weaponry. As I said, people in these riddles are capable of possessing guns for the sole purpose of suicide.

My surprise was at how a greenhouse could be related to a person dying from asphyxiation.

You should not assume that asphyxiation is the only way to die from a lack of air. Read through the answers above carefully, and I think you shall find what you are searching through.

[FinnKaenbyou]

The man was in a submarine and saw someone drowning?

[Fightest]

Oh, go on, next half-silly theory:

The Prestige scenario:

The man accidentally sealed someone inside a waterproof (or other liquid-proof) container, and filled the container up with such liquid. He realised his negligence upon observing the contents of the liquid through an observation window.