I think "evil" is primarily a word we use to describe things that we hate and fear. And we hate and fear things that we think will cause us pain or suffering.

So a logical, evidence-based definition of evil that I like to use is this: evil is suffering itself. I've expounded on this idea in another thread: https://nebulasaurus.medium.com/i-consider-myself-an-atheist-or-at-least-i-tend-to-be-much-more-sympathetic-to-the-atheist-96e1db557c0e

As for why people act in evil ways, I agree with you. If you look hard enough, you can always find a reason why someone behaves antisocially - or socially, for that matter. Judging someone as "good" or "evil" is ultimately just a playground for bias. Whether we believe someone is "good" or "evil" overall ultimately just comes down to what we want to believe about them, and which inputs to their behavior we choose to emphasize or ignore.