He's under the impression-

Mycroft was speaking hypothetically, but Sherlock had already established that John had the right about his behaviour. It was more surprising than painful to find out that his brother had given aid to his deadliest enemy. Sherlock thought it best to keep the conversation hypothetical, to allow Mycroft to avoid any possible display of feelings; emotional honesty was not in their shared vocabulary.

"And why are you supposed to have done that?"