Once a cheater always a cheater?

Yep. And I know that from personal experience.

The sort of person who will cheat either doesn't think their promises need to be kept or has a huge impulse control problem. The latter is nearly impossible to change. The former takes more work than most people can muster.

Yes, some can change, but before you trust them to try, you'd better be sure they're worht it an other ways.

Yupski

They can change, usually isn't worth the other party trying to change that person though.

Yes.

Imho