In S3E1, Sherlock didn't stop and really hurt Mycroft. He could have strangled him, punched him, bruised him, whatever. But he hurts him, and whatever he does in his drugged stupor, he doesn't remember when he comes down from the high.

Sherlock doesn't understand why his brother has suddenly become so wary of him, fearful, almost. John has to tell him what happened, and he is devastated. How will he make it up to Mycroft?