On the other hand, the guys who got Danny's sister killed didn't just pull a one-time crime, it's implied that they did that sort of thing regularly. So if Danny hadn't killed them, they would have never stopped getting countless other girls addicted to drugs, raped and killed.
The cycle of suffering and death would have continued anyway, even if Danny hadn't killed them, because she wasn't the one who started it and they certainly wouldn't have ended it on their own. The "if you kill them, it will never end" argument only works when the people you want to kill actually stop doing whatever it is that they did to make you want revenge.