Snape is a good guy in the same way that ice can burn you.
It's completely counter-intuitive and not the usual state or inclination of the thing in question, but it can happen.

Yes, Snape contributed in some ways to the same cause the good guys supported. He ended up in that situation by originally being one of the bad guys.

In rare situations, ice can burn you... but it's useless to try warming yourself by a block of ice or using it to light your way.

I might be losing my friggin' marbles, here: The neighbor's rooster shuts up for five seconds and the coyotes start howling. Hey, nature, STFU for a second, why don't you.