Oh yeah, something someone else pointed out to me:
1. Vriska is blue-blooded/text/etc'd, and at one point was missing an arm and an eye. (y'know, like the Blue Lady doll Jade was afraid of)
2. Gamzee is a
harlequin clown. (John hates his dad's apparent obsession with same)
3. Eridan is a wizard. (Rose [PASSIVE] AGGRESS her mom's wizards and stuff)
Each of the evil/killer trolls has attributes of something one of the kids hates with respect to their Guardian. Possible candidate for the fourth evil troll:
Terezi, who likes plush dolls (c.f. Smuppets).
Somebody else mentioned: Eridan has a HATE!bone-bulge for Rose. He didn't have to kill Kanaya; he could have just absconded, since it's clear that she wasn't remotely a threat. He's trying to stoke caliginous feelings in Rose, knowing about their friendship, and he's probably going to boast about it to her.