Honestly the way some people go on it’s like they think all Snape did as a death eater was bake Voldemort cookies. He was one of Voldermort’s most loyal followers, he most likely killed and tortured a LOT of muggles and muggleborns. And did he ever show regret for that? No. Just that the girl he obsessed over died. Wow what a great guy, I really should show him more respect.
If that’s all you get when you see Snape, then you’re missing the point. It isn’t that he was a “good” person, the point is that he’s a bad person who did good things at the end. He risked his life in the most dangerous and extreme way possible, way more so than any other member of the Order of the Phoenix, in order to keep feeding Dumbledore information. Also, if you think he was Voldemort’s right hand man before he gave him the information about the prophecy, then I don’t even know what series you read. Not all of the death eaters were as bad as Bellatrix, this point is made in the series over and over again. If you’re looking for some kind of black and white morality system, then this isn’t the right series for you.