I'll see you that question (Yes, it matters. Dispute over a parking ticket? Angry about a mural? Upset the Principal may have an inappropriate relationship with his mother?) and raise you another question: Do you really believe it doesn't matter?
The reason it doesn’t matter is that this is not a question of motive but a question of what brought this person to the point where he no longer had a soul.
He was evil. The question is not the reason he did this but the reason why he became a monster. Why was he evil?