I have a simple definition I call the Grand Unified Theory of Evil:
Evil = delusion * conviction * power
Delusion = how factually incorrect your idea is compared with base reality
Conviction = how little you will try to find flaws with the idea, how little you will test it, how likely you are to implement the idea at 100% of the universe if you had total power
Power = Your ability to push your idea into the world
This model isn't a tool to calculate evil. It's a tool to reason about how to avoid evil. First you need to be not wrong. Secondly you need to be damn UNSURE about what you know and try it at small scale. And lastly you need to have just enough power to test the idea, and show that it works to others if it does, but not any more.