Yes, it's definitely partly guilt on Keith's part! He's knows just why Ewen is so stunned to be taken care of by a solider in a red coat and the knowledge that his own side has been so destitute of honor and humanity just makes him shrivel up inside.

I love how even in his current desperate situation, Ewen is so warm and trusting once he realizes that it's Keith who has come back to nurse him - it really shows the sweetness of his nature. It would be understandable if he remained as cold and standoffish as he could, keeping his guard up just in case, but the idea doesn't even enter his head. Bs pbhefr, gung jvyy znxr Znwbe Thguevr'f npgvbaf gung zhpu zber cnvashy sbe uvz...Oevatvat zber uheg gb gur uheg-pbzsbeg.