Whatever you do, don't kill a character if it doesn't relate to the story in any way... I found that ridiculous and annoying in Hary Potter 7 when like 1 or 2 characters died each chapter for no real reason besides "Voldemort is evil so he kills everyone" and "there's no 8th book so these characters are all throw-aways now".

Also, you can "kill off" a character without that character dieing. Like, that character could just leave or whatever. But you have to be careful if you don't kill the character because there's always the possibilty of that character coming back and that's not always what you want.