You can't kill your best friend for personal profit. Cases that allow killing a friend are quite rare in general. By definition a friend is someone you feel attached to, who is not hostile, and whom you would by default like to help. The only case where you could kill a friend is when he wishes for that himself, and it would be thus an act of mercy, a coup de grâce.

So, you cease to consider the person a friend, not to mention a best friend, if you attempt to kill him for personal profit. Thus, you aren't actually killing your best friend, just a person you know.

Semantics aside, I wouldn't. It wouldn't be me anymore who did it, as it would assume a change of personality. Still, strange circumstances can change people a lot so nothing's impossible.