Just because people experience stress or a traumatic event and even fit the diagnostic criteria for PTSD does not excuse the acts of taking people's lives -- multiple times. (The morning news just reported that the shooter in Orlando told police that he "had been going through a bad time recently and he was sorry".) People ultimately CHOOSE how to respond to life events. Some make good choices and some make bad choices. People who make bad choices should experience the consequences for making that bad choice -- how otherwise, do they learn that it was a bad choice and they should never do that again. When we tolerate and excuse bad choices, we only make it easier for the next person coming along to make that same bad choice as there is no incentive not to do so.
I spent many years working with physically and mentally abused children in a mental facility and what I read and heard from them about events in their lives would curl all of your hair. I experienced a traumatic event and reading over the PTSD diagnostic criteria, probably could have fit into that some years ago. Compassionate PTSD, my azz! I haven't gone off the deep end and shot up a place full of people while yelling the equivalent of "God is Great".
We need to stop looking for excuses for every piddly-azz thing and stop "spinning" things to make them more politically palatable. Until we, as a nation, refuse to tolerate it, it will continue and we will continue to experience these kinds of events.
(okay, ranting is contagious!)
I agree with you when you say that people need to take personal responsibility for thier actions despite suffering from PTSD or any other mental illness. That shouldn't be a mitigating factor when defending one's actions.