The victims just had bad karma
I believe in karma and that we live many lives. As we die, after a while we start a new life without remembering the previous one. However. our karma follows us around. For example, let's say we killed someone in our last life. In this life WE would be killed to pay the price for that killing that we committed in our last life. All is in balance and everything happens for a reason.
My point is, let's say that you are a serial killer and you have killed a few people. Why did you do it? Perhaps you're insane, a maniac? NO. It meant to happen from the minute you were born because that's what your karma is. You have no choice. You must do it, it's inevitable and you can't change fate. You do it to clear off your karma and the bad deeds that you did in your last life. You might ask: "How the hell would you clear your karma by killing others? Well let's see...
The victims (the people that we killed) also had bad karma. Maybe they were rapists/killers/ so that's why this happened to them in this life. It's a circle. We punished them (we were chosen by a Higher Power to do this before we were born in this life) to help them (and to help us) to clear/better their karma for the next life. Again, everything happens for a reason.
Remember how Paul and Karla met? It was a complete accident. She lived in St. Catharines and Paul lived in Toronto. Not too far, but not exactly down the block either. Toronto is a major city and yet, they've met. WHY? Again, because it was meant to be and because they've had to work on their karma together.