"I don't think that someone said "sir, this is private property and would you please leave" would have set him off."
Heh, actually, some folks have gone off on other people for far less than that. I have seen people beaten savagely for nothing more than saying 'excuse me' to get past.
Why do you presume that anything the victims did had anything to do with what that lunatic did?
There is no necesary connection at all.
Some people are just vicious freaks and should be put down like rabid dogs.
I have not seen that and it is outside my experiences.