I will see if I can remember the source, but I DO know there was once some cursed artifacts that if the characters took them, each day the possessor had to make a Wisdom check/save DC 15. With each failure, the DC increased by 1. If they had three failures before three successes (similar to Death saves, maybe where they got the idea even...), they become Chaotic Evil and an NPC. If a character becomes evil, it is a greater story line to try to help them.
Also, as a DM, I allow Lawful Evil characters in my games. They have never been a problem really since Lawful Evil will work with the group and can be convinced to even help out if you present it with the right angle.