How can any human have the courage to kill another?

_ believe this one of my favorite questons ever, reason being is I've always wondered what makes people snap like that also, but as you see in the past it was believed to be witchcraft,then posession of body and soul, then it was family fault, no its got a name, bi polarism, wait ten mor years then it will be called somthing else, but I believe that some of us arw just bad in nature you know and whatever causes that snap we will never truly know till it happens but we are all born with a instink to protect just like we are alsoborn with a killer instint just like I believe that there some people without souls that cause the damage of ceareal killers because you really and truly can't have a soul to comit that kind of act you know, anyway good queston