I'll answer this one and ignore your other 2 questions because they're really similar and would get pretty much the same answer.
People consider Snape a bad guy, or at least partly a bad guy, because he did join the Death Eaters. He was a genuine Death Eater originally. It wasn't like he pretended to join them with the intention of spying. We don't know what crimes he committed during those years, but we can guess they included murder and torture. He was also fascinated by the Dark Arts and he did believe in blood purity.
I also got the impression that he wouldn't have cared or changed sides if anyone but Lily had died. While some people find that romantic, I see it as selfish. Another person's death wouldn't have mattered.
Also, he bullied students as a teacher and favoured the Slytherins. Other teachers, like McGonagall, weren't biased to the same extreme. I don't buy the excuse that he needed to act that way to keep his cover. He was doing it for years before Voldemort's return!
Professor Snape is considered a bad guy due to a couple of things. assuming that you have read the books you can see how badly he treats Harry Potter (he does this because of how much Harry reminds him of James who used to pick on Snape and married the girl Snape loved)
Another reason people believe Snape was a bad person was because he worked for the Dark Lord and was the person who told Voldemort the prophesy betraying Lily even though he didn't know it.
Mostly people believe Snape was a good guy but most of the reasons he was nice was because he wanted to make up for the fact he betrayed Lily.
Also the way J.K. Rowling writes the books leaves the suspense of Snape's loyalty throughout the books so you never know if to trust him or not.
Personally I believe Snape had the right morals but the way he treated my favorite characters means I will never like him like many people do but I don't hate him as much as others do. Snape was a nice guy who made lots of wrong choices.
I hope this answers you question, if not I'm sorry but I tried
Please ignore any spelling or grammar mistakes as I am not that great at writing.
