Dark magic is not evil, and light magic is not good.
If you want to become an Auror, you need to have knowledge about dark magic to defend them. If you want to become a healer, you need to have knowledge about dark magic to cure the victims.
If you use light magic to hurt people, then light magic is evil. If you use dark magic to cure people, then dark magic is good. The good or bad is how you use them.