I agree with your take on this, and I think this is one of the things that the movies (which I generally enjoyed) got wrong, or at least didn't make clear enough. As great an actor as Alan Rickman was, I felt he was miscast as Snape simply because he was too old for the role. In the first book he's not more than 30, and he never lives to see age 40, whereas Rickman was in his mid-50s when the film series started. I think Snape's young age is important to understanding his behavior, his choices, and his sacrifices.