It's more than she'd assumed. Could it have been enough?

If she'd stayed, might she have been able to talk him into a truce for some amount of time? Long enough to get him to see a way forward that didn't involve more blood on his hands? Or hers, for that matter?

The look she gives him is infinitely sad. There's not much she can say to a possibility that no longer exists. Even if they could forgive one another, Crait had shifted the balance of things. She couldn't bring the former Supreme Leader of the First Order back to the Resistance the way she could've returned with the General's son who had broken free of Snoke's command.

"If I'd joined you," she begins slowly, a part of her already withdrawing in anticipation of how much this is likely to hurt.

"If I'd taken your hand, what would you have expected of me?"

Any of the ways he could answer is likely to hurt, but finding that she's made the wrong choice might actually break her.