Over the course of a career? Probably a pretty significant amount have anger issues at some point. I started having some anger control issues after dealing with a child shot in the neck, took advantage of our program for such things, and moved on. Being angry and shooting someone willy-nilly aren't the same thing.
First, I think that the article conflates "anger issues" with "propensity to violence". Second, I would probably be worried about the psychological health of a police officer - especially one in a major metropolitan area - who did not suffer from anger issues at least occasionally. How can you deal with that much crap, and not have it impact you?