There's a thoughtful hum - telling doctor Bashir exactly what had transpired was likely to lose him a potential ally. The man was not unintelligent - he would have easily picked up on Khan's wariness, his hostility towards Starfleet. He's made no attempts to hide it.

"You wouldn't." Because he can see now that Julian is a healer first. The loss of life that no doubt resulted from Khan's actions would appall him. "My ship was too damaged to sustain orbit. I was disinclined to attempt to save it."

Which is, perhaps, the most delicate way of putting 'I crashed my starship into a planet in a fit of suicidal vengeance' ever.