Is there a limit of how evil can a person be? Yes, everything is bound by law of physics.

"The fierce tigers don't eat their cubs." No matter how evil you are, you must protect your offsprings, because if you don't, your evil genes extinct, thus no more evil in this world. This is the law of natural selection

You can not be the enemy of the world, because physically impossible, your total force and energy can not match a billion people, you will get killed sooner or later.

So if a evil person exists, he must command a force that equal to the opposite force that try to destroy him.

Evil is just ... stealing energy from other people, but you must be good to your allies, allies are your hands and legs, your body, an evil person still needs his body, can he be so evil that he destroy his own body? I think not.

So there is a limit on how evil a person can be, it's not in the head, it's in the environment, the relation with other factors.