It's hard to pin down, it's just a hint, mind you.
Celesto probably knew that Dom had captured Barnet. He also probably knew she was in jail. So, of course, she'd be punished for her actions.
Yet he jumps in and decides to go all vigilante on Barnet to 'amend', despite her upcoming punishment.
He didn't give a mention to the person who hired her, who would be just as culpable for the deaths and assaults.
It's just... 'hurhur, person BAD, me kill, fix world... dom not happy? DOM NOT HAPPY. ME MAD AT DOM. DOM GO ELSEWHERE.'
Well, he can just do a 'see the truth' on her soul. Or just disarm her mentally and help her work through her issues.
Why doesn't he leave? Well, he's already there. He already sees the problem. It's not in his nature, now, to leave when he knows there's a potential fix in the works.