Professor Snape
I understand that, completely, and that concern is one of the reasons we'd like to have your help now rather than if something were to go wrong. Should, for example, Voldemort come through the tesseract, it would be useful to have an understanding of your magic on a deeper level, as well as to possibly train SHIELD agents in how to deal with potential encounters. Assuming you have any potions that use ingredients available on this planet, it would be beneficial to have samples/antidotes/reserves.
No, evil is considered the antithesis of what we do.