Is it bad if you keep seeing yourself trying to kill your bullies in your sleep?

Only if you are planning on acting on your dreams!

There's nothing wrong with having dreams of any nature. Perhaps this dream represents the frustration and anger you hold against them, which is understandable. If the bullies are a big source of stress/concern to you, they may very well appear in your dreams. A confrontation may be in order, or you might need someone to talk to and help you with your situation.