Wanna know how it was supposed to end? Fazekas spilled the beans in this interview!

> As for Sam and the question of who his father really was, this one’s a doozy: “If you remember, Dad wasn’t dead. And there was a whole reason why he wasn’t dead. Basically, the whole premise that never actually had the chance to come out was the idea that Dad was a demon, who made a deal with the Devil. He fell in love with Sam’s mom and wanted to marry her. So the deal was, fine, you’re not a demon anymore. He was never fully human, either, which is why you can’t kill him. So, in the pilot, when he said he was really sick and made a deal with the Devil, he wasn’t 100% lying nor 100% telling the truth. And this is the reason why Sam is special: Sam is part human, part demon.” > > Boom. > > “Sam was led to believe he was the Devil’s son, but that wasn’t true. That’s not the reason Sam had powers — the powers came from his father. His father was really his father. And part of his deal was that he was never really allowed to tell Sam the truth, and that’s the reason why he says to him ‘there’s more to this than I can tell you.’ And part of what we tried to get out of all of this was that Sam really is special. That’s the reason why, at the end of the last season, Steve comes to him and says Sam is caught between good and evil. What we hoped to get out of the third season was what part did Sam have to play? Sam’s whole existence, and the reason the Devil was paying so much attention to him and hanging out with him so much was because there was a reason: Sam was half-human and half-demon, and the Devil was really worried that this kid was going to be his downfall. If you look at what the Devil is doing throughout the series, he’s trying to tempt Sam to be bad. And he’s trying to tempt him to embrace this, that what you’re doing isn’t so bad. The whole point was that if he can turn Sam bad, the Sam isn’t a threat anymore.”

There's a little more besides in the interview, definitely worth a read.