Evil is a very broad definition. What defines evil? Is it an act, or is it the choice to do the act? How can something be called evil if it knows no other way? Is not the true definition of evil those who can see more than one path of any situation, and still go down the road of murder and mayhem?
Edit: Also, unless imps are in another town as well right now, i doubt for the longevity of this new Paladin girl.
Evil is a choice. However, creatures bred to perform evil acts will perform evil actions, no matter what. They might change if offered another way, but Sarin is more concerned with protecting good, than with redeeming Evil.
Heh, Sarin might surprise you. Just wait. ;)