In SCC it is hard to see what keeps driving Sam after finding Sarah alive, but the whole ordeal is occurs over what appears to be a short time frame and the level of the threat to the U.S is more immediate than Sam has ever seen. So with him still being off balance from all the mind fudging he goes through in C, I kind of get why his decisions are perhaps erratic or contradictory some times. To me, it was the episode where we had the most chance of losing Sam to the dark side
I never forget that Sam, no matter how endearing, is a man that kills for a career. He IS a killer, whether those actions are justifiable or not, the effect on his psyche from doing it for so many years has to have a lasting effect.
I can understand him getting to a point where he wanted to stop and I can also understand him easily slipping back into it, or even going over the top with it.
I agree that is character is largely based on how we the Player, played him, regardless of the comments he made during those play throughs about not wanting to kill or whatever. He made plenty of threatening comments to enemies about killing them in a very dark humored way too, and it comes so easily to him.