The US Supreme Court unanimously decided that a cop can't be sued if a they shoot or kill a suspect without having heard other officers issue a warning. In other words, if a cop shows after other cops have already established some intel on the suspect (he's armed, isn't armed, on drugs, etc.), and the cop shoots said suspect, the cop is automatically in the clear on any charges.
Unless of course, there are prior cases of the exact same situation, where someone suited and won. And if you look hard enough every case can be different. Noah Feldman, professor of constitutional and international law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter give his analysis on some of the possible outcomes in this Supreme court ruling.