As some of us know, in the intro to The Slim Shady EP, Em is possessed by a ghost-thing named Slim Shady that he killed before.
Is it possible that Slim Shady was the guy described in Backstabber on Infinite? Maybe Marshall killed him, and when he sold his soul for a record deal, Slim saw it as an opportunity to get revenge by possessing him.
I'm probably reading too much into this, but maybe Eminem thought it that way. Who knows?