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If you're looking for a character archetype, there's Lovable Traitor.
Heel–Face Revolving Door when they fully flip between heroism and villainy.
Chronic Backstabbing Disorder may result in Forgiven, but Not Forgotten if there's a second occurrence, with subsequent reoccurrences resulting in eventual erosion of trust.
@OP: The closest trope that combines Chronic Backstabbing Disorder and Easily Forgiven is Why Would Anyone Take Him Back?. It's a romance trope, but it could just as easily apply to someone who's repeatedly forgiven, despite having a history of repeat betrayal.
Chronic Backstabbing Disorder meets Easily Forgiven.
1. You meet a guy.
2. You trust him.
3. He betrays you.
4. You end up being okay.
5. The guy shows up again, apologizes, and begs for forgiveness.
6. Go back to Step 2.
This, constantly over and over, even though the Heroes shouldn’t trust him