Not only do they carry guns, they are trained to automatically assume that everyone they encounter is out to kill them!
Imagine how paranoid and afraid that'd make you after a while!
Given that, it's not really surprising we have police officers killing someone who "pulled a gun" -- except that then they discover it was just a candy bar.
I've got a few friends who are police officers and I wouldn't want to do their jobs. Of course, my LEO friends are all older, smarter, and, due to experience I suppose, more "laid back".
Nowadays the first thing drilled into a new police officer's head is that "everyone wants to kill you".