Catching a shooter BEFORE the act is impossible, as there is NO law against "bad intentions", first they must be charged after the fact and declared guilty of the crime, before anyone can confront them.
Leaves us naked and powerless with only a reaction to our grief. UNLESS we are armed and given a chance to defend our self and other innocent victims. There is nothing written on one's forehead allowing us to know who is dangerous, parents believe "not my kid", and will cuddle them. We have a society that wants abortions, yet demands we end capital punishment. Are we that lost?