I don't think the story proves that Snape was GOOD at all. Loyal, yes, but Good? He loved Harry's mother (or was infatuated with her) but he clearly did not love Harry, because, in his estimation he was more like James than his mother. Can you imagine how
Snape would have been as a step-father to Harry, had Voldemort killed only James and Snape had gotten his wish...and assuming that Lilly would have had him? Snape frankly told DD that he'd wanted Voldemort to kill James AND Harry so he could have Lilly
for himself. I'm sorry, no way does that EVER make Snape a good guy ... in love or otherwise! I also don't think I'd want him teaching in my school