We already have those, they´re called ¨Cops¨ and as it turns out they aren´t exactly infallible....I´m not sure if I would want another 100,000 of them running around.
I´m sorry to hear about your friend....unfortunately, I don´t think we´re going to learn what we need to learn this time around, or the next time this happens. The US as a society is too good at avoidance to confront the real issues, like ¨what are we doing that causes people to snap and start shooting into groups of other people more than anywhere else in the civilized world?¨