In all the time that I have been a Satanist no one can actually explain what evil is better than my definition.
Evil is simply what you don't like -- Period. I am always evil by this definition because I don't care what you like only what I do.
Some mistake this concept for other things...
Some feel that vanity, aggression, torment or torture, murder, or other things are evil. They're not... We kill things all the time to eat them, and even if you were vegetarian you step on a few and accidentally eat a couple of spiders in your sleep. Why does it particularly matter if someone is killed for a government sanctioned reason, a religious reason, for resources, or something as simple as I freaking feel like it? In the end the net of it all is the same. These things only become bad per Se once you need a society of peons under your control so that you can harvest the rewards of their work. If your workers killed one another then you'd probably be losing the product they make or the money you make off it. Not too good for the powers that be is it? Any illusion that killing is anything but killing is simply that as is any notion that killing a person is worse than killing a chicken, or even worth more than a single chicken -- etc.. I think this probably had more to do with religious authority initially as it would be really hard to keep the flock if they were taking one another out like the old days. Demonize what you don't like... Naturally..