Sure, but I will also say: "If any one of those individuals was armed and trained, they could have defended themselves and his spree would have been ended." My response was directed towards the analogous case of a similar rule being applied in a different situation. Also, when did self-defense become vigilantism?
When you said you wished the man had been shot, you didn't say "I wish he'd been shot in the course of a crime." I'm willing to believe that that's what you meant to imply, but as an independent reader observing the conversation, that's not what I extrapolated from your statement. I simply saw a wish that a person had gotten shot because he had committed heinous crimes. That's why I interpreted your comment as potential vigilantism as much as potential self-defense.