You cannot kill an entire clan, unless it is a made-up clan. Then you must have permission from all the clan members. Example:

You want your character to be evil.
You want to kill another character's clan.
If their clan is made-up, PM them.
Ask them if you can kill them or their clan.

You have to have permission to kill another's character.

You can make multiple new accounts and kill them, if another player disagrees

Also if a player has not been on for over 30 days, you can kill them because they will be deleted anyway.