This is a complicated question, because in order to answer it properly, many other questions much be answered, like....
- Why did Dumbledore trust Snape in the first place?
- What was happening under the surface when Snape made the Unbreakable Vow at Spinner's End?
- How much does Snape know about the horcruxes?
- What did Dumbledore mean when he said, "Severus, please" on the Lightening Struck Tower?
- Was Snape secretly (or not so secretly) in love with Lily Potter?
Each will be addressed here in turn, but you should know I'm biased. Dumbledore is one of my favorite characters in all of literature. With all of my heart I want him to be as infalliable as possible, and if Snape is evil that means that Dumbledore screwed up big time. Although I acknowledge it's possible, I don't want it to be true, so I tend to look for reasons to prove Snape good rather than evil.