Snape is the kind of character where I don’t necessarily believe the author knew what they were doing with them or that they had decided but hadn’t thought of how to get there. There’s a lot of excusing for his behaviour which I don’t think he deserves because his redemption arc doesn’t exist or isn’t convincing enough for me in canon. I like your approach on how to change characters — I think a good writer understands that there is no one income and that changing history can happen in many different ways.