Don't blame it on movies. If movies disagreed with accepted culture, they wouldn't be popular. Qualified immunity and blue wall of silence exist not because of the movies.
Also note that Hans Gruber is not exactly an example of average citizen and was shown committing several murders (not to mention a row of other felonies including hostage taking) before McClane has even a chance to encounter him. This is exactly the case where police officer is justified to use lethal force - unlike, say, woman in her pajamas calling the police because she heard a strange noise in her backyard. Constantly confusing these two is exactly the reason why no sensible discussion happens in US public debate about it - it's either "police can do no wrong" or "always blame the police, even if the other side is Hans Gruber".