[It does sound bad, there's no denying it. But he had changed, it seemed. She's not sure how much she can trust his words, but he's admitting he now knows he was wrong before. And not accepting him when she has no issue accepting Finn would make her a hypocrite, she knows full well. And it's not as if he's the first, or the last, imperial defector to the rebellion's side.

Leia is silent for a few moments after Galen's explanation, studying him closely and looking incredibly guarded as she does so. Finally, when she speaks up, there's no smile accompanying her words--it's difficult to bring back the pleasantry of earlier now that Darth Vader has been brought up--and her tone is more formal than before, but Leia sounds very much as if her mind has been made up, willing to give him a chance knowing what she does now.]

I can imagine full well how that monster must have treated anyone he interacted with regularly. I'm glad you've learned your own worth, and decided to fight with us against the Empire. People with your kind of loyalty are always welcome with us.