The reason varies from killer to killer. Some men killin self-defence. That isn't really so bad. Others killin fury. That's bad, but at least there's a human emotion there.
Some men kill for pleasure.
That's sick, but again, at least there's some sort of human emotion. I used to think the worst kind of man was the one who killed merely because he was paid to kill. There's no emotion, no real reason - nothing. A man like that can kill his mother without blinking an eye, and the victim would never understand why. I think that's the worst way to die, not knowing why.