The Horror
Jeff wasn't insane. That's why he shot all those people.
When you get to the point when just thinking about it turns into just doing it, you'll ask yourself if you're insane too. And if you say 'NO', then I imagine lots of people are going to die.
Hell, being insane would alot easier for some of us. If we were insane, people might have sympathy. When you've crossed over to the dark side intentionally people just think you're a monster.
Maybe they're right.