Rey's stern response is met with a guilty look, one laced with uncertainty. Carlisle can only speculate on what might have happened if he'd done differently, if he'd agreed to Shadow's request or compromised rather than denying it outright.

And speculate, he has: "Perhaps, if I'd taught him to heal, he'd still be alive." He turns his glass as he caters to his nerves, inwardly wishing he had a few more drinks to quiet them. "He could have mended his wounds, or maybe he'd have not fought at all, having found some purpose for his energies."

Carlisle makes no mention of what that would have meant for him personally -- he cannot even consider it when Shadow is dead and he thinks himself at fault in some way.