Some people are just plain evil. A path with the evil one is just plain more fun than being a pious innocent person. When someone has evil in their veins, being a monster, commiting vile acts and the like become everyday things. Ah no, I think I shant tell you. To see it would drive you mad, to hear it would soil your soul for eternity and to say it would kill you.
Some people just figure it's cool to be bad.
While not originally in this writeup, during the course of response Jean-Paul Molyneaux had a truly diabolical thought. Hence, as you fell under the sway of greater and greater evil, your true understanding atrophies due to lack of use, and more and more of your sphere levels become infernal spheres. He was measuring this drop by the number of infernal points gained. Hence for Arete to drop from 3 to 2, you'd have to have gained 16 infernal points, the same amount of experience it took you to gain that level. This is painful and takes a long time.