It reads as if he pulled the chocks, started it up, did his own push-back
using reverse thrust, but then had things kind of unravel from that point.
Don't really know why this guy did not get airborne as he certainly was
qualified.
If you study the photographs accompanying this news story--
The Associated Press: Official: Colo murder suspect tries to steal plane , it appears that he didn't actually "push back" at all. From the end of the jetway, it seems as if he only went forward, passing alongside the terminal building on his way into the parking lot in front of the terminal.
One
might speculate that he damaged the wing almost immediatly, in contacting the jetway, and realized that he wasn't going to get off the ground, so he just proceeded forward to his ultimate demise, but far be it from me to speculate...