To be brutally honest, I don't blame him.
He had a hand-gun and all he knew was that there was a shooter with something far more powerful.
According to an interview I read with an ER doctor, a 9mm wound is equivalent to a bad stabbing, but an AR-15 round leaves an exit hole the size of a baseball and ruptures everything alongside its path.
Some fool with an afternoon shooting paper targets and a head full of Die Hard movies is going to feel indestructible and would take on any shooter. Somebody with training is going to know what could happen if he charged in without backup.
This guy is going to spend the rest of his life haunted by those deaths, but if guns were properly controlled, and if gun violence wasn't constantly glamourised in the media [games as well as movies], and justified by the NRA, a huge number of lives across the US would not be lost.