there would be no discussions about anything.
In this case, I believe the 'why' is about what makes a person's brain make them want to kill someone, or make them capable of it. It may boil down to chemicals in the brain, diet, experiences stored in memory, or all of the above...it's a combination of factors meeting at the moment of the crime.
My thought is that if you want a rational answer, you will not always find one and that's the frustrating part about this type of discussion.
Accidental killing, like running someone over in the street, is simply bad luck.
I find that the legal definitions can actually hinder the discussions on 'why' they occur...but, to each his own.