There seem to be two distinct issues here:
- Did Mercer act with integrity
- Should Historical events continue to be investigated.
It seems to me that whether he resigned or was sacked, he has acted with integrity. He believes that the historical cases should not be investigated and that Boris signed up to this. Regardless of whether he is right or wrong, he has decided not to bite his tongue to remain in office, and in my book that shows integrity.
As for whether the historical cases should continue to be investigated, things are less clearcut. IMHO they should not, for a couple of reasons, A) it seems to be a rather one sided process B) I seriously doubt that a better understanding of what transpired and why can take place after all of this time.
Reasonable people can differ on the latter point, but I do believe Mercer has acted with integrity.