That's not what happened to the Davidians. They were acquited on all capital charges. So (assuming for the sake of argument that they actually shot any 'cops'), you're flat-out wrong. There are grounds for shooting 'em. I support the death penalty for killer cops as well as for cop killers.
But you have brought up a serious problem: how do we keep the Feds in line? think Horiuchi, who was properly charged in a state court with murder (IIRC), and was given the all-clear by a Fed court.
That's not what happened to the Davidians.
Are you saying that they were guilty?