Well, the guy he took the gun from was a few fee away, and the other armed assailant was still busily shooting civilians. The police didn’t show up for half an hour and then they hid til they thought it was safe. It appears the government have not only disarmed the citizens, they’ve castrated them to boot.
The guy would have no idea when the police would show up.
I haven't seen any news of how the shooting happened, any time line or map of where the shooters were in relation to the man that disarmed the son. So, I don't know if he could have taken a shot at the father.
But, consider was this man familiar with firearms.
Consider he lives in Australia where he very well could be prosecuted for shooting the murderous father. (I don't know if that is the case, but it is possible in some states in the US).
Maybe we should celebrate this man's bravery for disarming a man in the midst of committing mass murder instead of second guessing his actions immediately after.