I don't think there is anything we can do to the shooter afterwards.
The damage has already been done. Taking one more life isn't going to make things better than they were before those innocent people were murdered.
True justice is not getting back at the people who commit crimes afterwards, its successfully preventing these things from ever happening ever again.
Saying "Lol lets just kill them, problem solved." is not a solution. Haven't you considered that the people who do these sorts of things already know they're crossing the line and essentially ended their own life? These psychos don't care what happens to them afterwards.