But WHY would we want to know this? I think it's so we can gain some sense of control of the phenomenon. Pretty much everything we humans think and do is for the sake of gaining either actual control, or imagined control of the world around us. Because we humans survive and thrive based on our ability to control/anticipate our environment, (including each other).
Again, WHY do you think we want to understand this? I think it's the same reason we want to understand anything, or at least imagine that we understand: so we can anticipate/control it. Victim-blaming is a very common and easy way that we humans imagine that we have control of our circumstances, even when we don't. And make no mistake, we will choose to imagine that we have control even when we don't, over having to face the frightening reality of our actual lack of control.
So when something like this happens, we all become frightened, because it's a glaring reminder of our vulnerable lack of control. And we respond by searching for any means of regaining that control, real or imagined. And that's WHY we are all so 'fascinated' by these events. It's also why we have such a hard time recognizing even the most basic solutions, and implementing them. Because in truth, we are all 'freaked out' by these blatant reminders of our constant vulnerability and our lack of control over the circumstance of our own lives.
As for why people want to understand, it may also be due to wanting to understand the human mind so as to support and implement measures to improve the human condition. It's a way of learning from our mistakes. For example, some may recognize that crime is a symptom of poverty, abuse, addiction, poor education, lack of opportunity - things that society has a vested interest in addressing and changing. That's how human society can progress - and much of it has been through trial and error.
In any case, wanting to understand why things happen is better than not wanting to understand. And there are those who probably don't want to know or understand why he did it, but would rather expend their energies expressing rage and contempt for the perpetrator - which is certainly well-deserved, but also a waste of energy and intellect. I feel it's better to put that energy to use to try to logically analyze just what is going on and try to see the cause and effect between events.
In regards to victim-blaming, I don't deny that it exists, although I think it's sometimes exaggerated. I've known guys who left their wallets on the counter in a crowded bar, come back hours later and find that it was gone. Even they would acknowledge that it was stupidity and carelessness on their part. Obviously, the thief was to blame, but pointing out the mistakes of the victim in order to learn how to avoid it from happening again should not be considered out of bounds. Others may also learn from those mistakes.