if someone robs you and you kill them is that first dergree murder

There is a chance it wouldn't be murder at all.  But a question like this is going to be very fact specific. Under the scenario posed in your question, if you are looking to raise a self-defense claim, you will have to persuade a jury that you acted in self defense, so keep that in mind.  Furthermore, self-defense is a last resort in a situation particularly if you were to use deadly force, such as a gun.