Yeah this one's a weird one. It depends on who said it or how it was said, but but best way to explain is that whoever said it is facing consequences. Say I failed a class and my parents got my report card. I'd say to a friend, "I'm in for it," meaning that I'm going to be in trouble when I get home. Does that make sense?

I'm in for it. The person believes that the other person, Hardin in this case, will seek out to punish, retaliate or get vengence on him or her. A silly example: If you break the window on my car you'll be in for it because I'm going to slash all of your tires.