"What I did, I did without choice in the name of peace and sanity." he said facing his Eleventh self who had Clara clutched in his arms, knowing that The Doctor wouldn't understand, wouldn't see past the hatred of him that had been fostered several regenerations ago. Hatred that he'd deliberately created.
It had been pathetically easy to convince the Master that he was the distillation of all of the Doctor's evil. In many ways he was, seeing as he was the last, the end result of all of the worst the Doctor had done. The man who'd tried himself and found himself wanting.
The truth was, he had done what he had done in the name of peace and sanity, and there really had been no other acceptable option.
Considering the way he had been heading, especially in that Sixth life where his hesitance to kill was reduced with each passing day, where he had stood and killed with his bare hands for no more reason than revenge, there was no telling what he would've become if he hadn't brought himself short, dragged himself to heel, and forced himself to face what he could be. Forced himself to see that if he didn't stop and check himself, he could become the very thing he'd spent so many centuries fighting against.
There was a darkness in his Eleventh self's eyes, a darkness he recognized as it was nearly as deep as the darkness in his own, but it was nowhere near as bad as it could've been for a man who'd been through things that would drive lesser beings to madness, beings who had nothing to firmly hold onto as they tried to claw their way out of the dark.