If you wonder how someone ends up in an abusive situation, the answer is usually "boiling frogs." (The parable https://en.wikipedia.org/wiki/Boiling_frog not the reality.)

If you find yourself wanting to say, "Oh, that [abuser] seems too nice to do this!" I want you to KEEP IT TO YOURSELF. The most skilled abuser knows exactly how to seem like the nicest person ... in public and during courtship.

Offer moral support if you can, and please don't say "Oh, I can't believe that!" Please.