The shooter wanted to take his gun into the office and use it to fire bullets at as many people as he could. The security guard felt strongly that this was not a good idea.
Words were exchanged and the situation escalated until it got out of control. Not much more to it than that.
Now please move along. There's nothing to see here.
I do feel that a reasonable, civil compromise was in order here. The guard should have let him in if he promised to shoot only a few people.