Well, sometime in season 1 or 2 Erin was held at knifepoint, and in season 3 she was almost raped, and Voight didn't do anything worse than what he's done to perps before... lol. But if they had been able to go further in either situation, yeah I could maybe see Voight taking it further than we've seen thus far. If Erin was just hurt and didn't die, like Justin, I don't know if he would go so far as to kill whoever did it, but... just my speculation.