i agree with you he most likley thinks that sirius is still guilty for harry's parents murder but i don't think that snape is sorry for the potters death he is just trying to shove it in harry's face that his parents are dead and everthing and that he is trying to become friends with siruis after he did that. But i don't think that james did anything for this to happen except save snape when sirius tryed to get him killed because i don't think that sirius is guilty i just think that snape has that crazy idea in his head that sirius is.