There are two characters, I might not kill either of them but I'm considering killing one because they're the two people closest to another character and sending her off on a murderous rampage because someone killed her sort-of-brother/friend would be entertaining.

There's the sort-of-brother who's not necessary to the big plotline but is my favorite character in the book, and he does serve a purpose. He's terrified of failing and letting an ally get killed, and would probably die stopping someone else from dying.

Or the character who is necessary but I can kill him after he's served his purpose. He would probably die in the major battle at the end.

Should I kill either? If so which one?