For awhile there, cop-hating liberals and libertarians had themselves a loophole that made it easier to come down on cops who used force in self-defense. According to the so-called "provocation doctrine," if police had made their way into a situation without satisfying every legal technicality (such as entering a home without a warrant), then they were not necessarily allowed to use deadly force to defend themselves even if another party threatened them with deadly force.
In the case in question, police had indeed entered a home without a warrant and encountered a person wielding a BB gun that looked like a real gun. That prompted police to shoot, but not kill, in self-defense, and initially a court ruled that the officers had no right to use force because they shouldn't have been in the house in the first place.
But that goes against all jurisprudence on this issue, so much so that even the way-out liberal 9th Circuit found in favor of the cops, and so much so that when it got to the Supreme Court, even the liberal Justices joined in making the ruling unanimous: