How can an Angel, or any entity, become evil?

They chose not to honor Jesus as God's infinitely beloved Son. They rejected the Son in favor of their own self proclaimed "righteousness". They thought they were good enough to merit eternal life without submitting to Jesus.

Since nothing supernatural exists your question is moot.

It's a mystery as to how any entity could become evil.

Through the self-justification of their own thoughts, actions, and intentions.

There is no such thing as an angel - or a demon, or anything like that.

It's just outdated superstition.

He does not have to do anything. It just takes one of us who doesn't like that entity or what it does.

Evil is not a trait of something or somebody. Evil is an opinion about something.

Think about Robin Hood. Whether he was a good man or an evil and dangerous criminal does not depend on what he did, it only depends on whether you see yourself as a rich person or not. It is your personal opinion.

...it makes for a better story.

Free will

Well, you seen how they do it in army training? They just yell at you, and tell you you're a piece of worthless crap, until you get so mad, you want to kill, anyone.