No I get all that. But Gregson also could've lost his job (or been severely punished at least, idk how discipline works in police stations) if Sherlock told anyone about the planted evidence mug on that serial killer. True, it was all the partner's fault, but even she thought Gregson knew, so who would really believe him in the end? Oh ~I didn't know~. I feel that Sherlock had stored up a lot of goodwill in Gregson after that, but aparently it was all forgotten in last night's ep.

(if anyone gets to punch Sherlock it should be Joan, lbr)