You're a demon. A demon that has been obsessed with Sam since Lilith began breaking seals. But, that was years ago. You, somehow, by some freaking miracle, managed to get Sam to fall in love with you just as much as you had fallen in love with him.

You had always been overprotective and possessive of Sam. You can't help but make dark, twisted comments about keeping him forever locked away and out of harm. You don't ever mean any harm by these. You're a demon. It's how you express love. Sam, in a way, has always found them endearing. Sam is the most precious thing to you.

Dean has the Mark of Cain. He was killed, and the mark brought him back as a demon. Cas and Sam searched for him for months before finally finding him. They captured Dean, and Sam tried to cure him whilst you and Cas were out. It went haywire. Dean became close enough to human that he could slip out of the devil's trap, but was still demon enough that attempting to kill Sam was not an issue.

You finally got back to the bunker. The alarms are going off. The whole thing is flashing in red light and furniture is turned over...