Your typical police department will use a mental health screening process for new recruits. I'm assuming the LAPD is no different in that regard. There's also a screening process for military branches, including the reserves. So he went through at least two evaluations in his life and sailed through into positions which let him learn exactly how to kill people with precision. I'm not sure if this says something about how virtually anyone can turn given the wrong conditions or a desperate need to revise those tests.
He's crossed a line most American criminals are supposedly reluctant to look at: not only targeting police, but going after their families. (Overseas, it's frequently considered a good way to keep the number of dedicated officers down.) It makes him that much less likely to be taken alive. It also means the victim count could still go beyond his own: that nearly happened already.
We train our killers well. We're about to find out just how well.
The manifesto: http://www.dailynews.com/ci_22538933
I don't think this one is going to suicide on his own.