Okay, I feel like the whole "What if he was armed" argument is being viewed backwards. If he's armed, a coach should DEFINITELY not try to stop him. It is not a coach's job to deal with someone on the field like that. He immediately becomes both susceptible to harm by the person on the field AND liable for any harm inflicted on said person. So when everyone says "what if he had a knife," that's when he ABSOLUTELY should not be trying to tackle the guy.
To give you a corollary: If I am on the inpatient psych unit and a guy elopes, I amd instructed to NOT try to stop him/her myself. If I try, I am at risk of being harmed AND if I harm him, I'm going to lose my job for harming a patient. Trust me, my first night ever on call, I tried block an exit when a psychotic guy eloped from the unit, and I could have been seriously injured. I also would have been liable if I'd hurt him somehow. I followed the guy so we knew where he was, but it was security who had to physically restrain him. And I definitely learned my lesson. Now I don't even wear ties at work because they're a choking hazard.
Does this excuse the behavior of the guy on the field, absolutely not. But the coach made a decision which could have cost him his job. At the time, the guy was not running into people and was no longer involved in the play, and so the correct decision is to ensure player safety by getting players out of the guys way.
This isn't about morality or what the guy does or does not deserve. It's about liability and safety and legal process. It was a cowboy move that could have gotten him hurt or cost him his job and his financial security. Regardless of what you believe about our laws, they are our laws, and it is security's responsibility. If you want to harp on why security didn't get to him in time, I won't argue with that. But what the coach did was legally iffy at best.