Before my brother married his current wife, he has a long term GF who I couldn't stand. I used to stir the shit in their relationship.
I wouldn't say I justify it to myself now, I know I behaved like a shit. But maybe I just don't regret it? I still think she was manipulative and opportunistic, and he just didn't want to see it.
Every 6 months or so I remember how petty and mean I was in very subtle ways. Kill them with paper cuts kind of thing. I'm mortified honestly thinking of it. I am mortified but don't regret it honestly.