From my understanding, he was intervening to try and break up a domestic dispute in progress... don't know much beyond that. While there might be a set of circumstances that explains why he was shot several times in the back while calmly getting into a car (like that video of the lunatic chasing a cop around with a knife for a full two minutes before she finally has little choice but to cap him), I can't imagine what those circumstances could be.
If you find and watch the whole video, though... it's pretty fucked up.