He should be commended for his actions by the public, because what he did was a selfless and brave act, he is clearly a man who takes his job very seriously.
That said, he was in the wrong for not informing his superiors about leaving his post, and if another shooting or similar problem had happened in the university while he was away many students may have paid for it with their lives. I think the university needs to first look into the way he contacts his superiors in emergencies such as this, is it as simple as a call over the radio, or would he have to travel halfway across campus in the other direction to the office, and he should be lightly repromanded for his dereliction of duty, made to attend a short revision course on his duties and regulations, but firing him is too harsh a punishment in my opinion. They have just lost a valuable member of staff.