You thought it was actually experience points!
Too bad, it's more "genocide vs. pacifism" pretentiousness! (Damn, Toby, back at it again with the pretentious stats!)
It's not experience points, nooo. EXP actually means "Execution Points", which are points awarded for executing people. Of course, no execution is a good execution, so enough EXP means you go up a Lv.! And going up a Lv. means you're a murderer and, in Sans' eye sockets, you should die in a hole. Likely, this one.