Is it feasible for a character to kill his childhood best friend?

Depends on how well implemented it is. Maybe it wasn't intended. Maybe one of them pushed the other, and made the one he pushed hit his head off something real hard. Maybe they came to blows about something, and one of them struck the other over the head with something (a baseball bat, for example) and kept on hitting over the head in a fit of anger. That seems to be feasiable. Add a bit of spice to it and give one of them anger management problems, make them naturally aggressive or a bit off in the head.

Also, the idea for a character to kill his childhood friend isn't that strange. I saw one thing where a kid, who was about 19 or so, murdered his father becuase his father wouldn't buy him the car he wanted. The kid woke his younger brother up, took him to a neighbours house, then went home, shot his dad and threw a stone through the window to make it look like a break-in.

Also, having it in a story can help serve as bit of spice for the character. Killing someone is bad enough, but killing your best friend? Make it so that if people know he killed him, they react to it. In a story I'm writing right now, one of the main characters (Alistair) murders his brother (Alexander) by shooting him in the head-Alexander deserved it though, becuase earlier in the story he betrayed Alistair and shot him in the head. Alexander begs Alistair to forgive him, and Alistair aims his gun, and before firing says "Don't worry. We're even now."

 

I agree with most of the posts above. People kill people. Parent's kill their children, lovers kill their lovers, children kill other children. It's a sad but undeniable fact. What matters here is how you write it. Have a motive. Have an internal struggle. A community backlash. If you've conceived this idea then surely you have a reason for it. Make sure the reader knows what it is.