That’s how Snape stands being thought evil when he is actually risking more than almost anyone as a double agent against Voldemort. The longer he can remain hidden, the more he can protect not only Harry but his own Slytherins, boys as needy as he was at their age. The Inquisitorial Squad showed him how easy it would be for others to turn them toward evil. He will do what he must to stay close and protect them.
