They say Sam sold him out in the end.

Threw his lot in with Hell. Sold his soul to the Devil, and swore allegiance to Lucifer himself.

I even hear he got hooked on drinking devil blood. It's supposed to grant... some kind of powers. He kept it from Dean for months: the addiction, and the truth about who he was working for.

I don't know if it was the angel, one of his buddies, something outta hell, or Dean himself. But I wasn't surprised to hear that Sam wasn't left standing when it was all over.