An evil person can also do good just because it's good. To be evil you don't have to be 100% evil. Someone who is 70% evil is 30% neutral and good, which is how you get those evil folks in the world that wouldn't harm children and instead protect them, and other good acts. That person is just predominantly evil, so gets the E tag on his alignment.
A realistic personality isn't going to fall neatly into say LG. That personality is going to be something like 55%(L), 25%(N), 20%(C), 75%(G), 20%(N), 5%(E) or whatever other combination ends up being predominantly Lawful and Good.