"I need to leave the hand behind." It's the most obvious mark of who and what he is. He can live without a hand.

Probably.

He'll make do.

"Pretend you didn't hear me plan or see me leave. Leliana will continue to keep you safe and see to it the Wardens are considered as they move forward." He's leaving. He's leaving and nothing is going to keep him here. Not Leliana, not Alistair, not anything.