I don't actually disagree with you. The use of lethal force was the mistake here, but the presence of force was justifiable. The problem is how easily one seems to lead to the other. What should have happened is the police realizing their force was unnecessary and then standing down.
Also, if someone were to call in, say, a terrorist threat or a bomb threat in Britain, someone would show up with guns there as well. You know, because Britain also has their equivalent to SWAT teams. With guns. That they shoot people with.