The psychology of the no-shoot is interesting. One moral heuristic seems to be that it is wrong to take an innocent life even to save other innocent lives.
This is instinctual perhaps so the risk ratio argument may not contradict it. So in arguments about campus carry - the risk that a good guy might kill an innocent to save many doesn't work for some.
Consider the scenario, where a shooter enters a locale. You shoot him before the massacre really rolls but also shoot some helpless person. Think that will look good for you?
In competition or FOF, shooting an innocent points out that it may not be easy to just take them out in a good shoot.
I remember my partner in an FOF exercise clearing a house, coming out from a room, saying - I just shot my son.