There was a reason he'd made Sam, and other people, promise to handle him when the Mark took over. As a human, what it did to him had been the worst possible thing. It made him hurt innocent people, made him lose control, and if he didn't give into it, he was the one who suffered. Which would be okay, except suffering just made it harder to resist and he gave in anyway.
Now, he didn't care. He liked the thirst for blood the Mark gave him.
"All the supernatural crap's above your pay grade, kiddo. Don't worry about it. Everything's worked out." Exactly the way it should.