Why was Mycroft indifferent about Mary's potential death and the possible consequences of Ajay's reappearance?

In season 4 Sherlock clearly says that Ajay is a killer and hints that he could murder Mary, but all Mycroft says is that agents die. Sherlock goes to Mycroft for help. He sees Mary as a friend. He also made a vow.

Mycroft should be interested in the sudden appearance of a guy thought to be dead. I don't understand why Mycroft doesn't see that Ajay may pose a threat. Ajay was tortured and held captive. He knows things. Probably he heard and saw plenty. Maybe he’s working for the enemy. To me it's obvious he might be a threat to both Mycroft and Lady Smallwood. All thay Mycroft does is tell Sherlock you can’t keep all the killers away from her, which I found surprising. No wonder Sherlock angrily says not on my watch.

He knows that Mary is John's wife, a mother and also Sherlock's friend. Mary may be an assaasin, but she's not a baddie. Mary has shown she's on the good side. Sherlock trusts her as does John. Misunderstandings about her behaviour are explained in the series. Lady Smallwood headed AGRA, Mycroft disbanded it. Tbilisi was 6 years ago, the lady proved through her behaviour she’s on the good side. Questioning her can wait. Mycroft allows Ajay to almost kill Mary until the police kill him and Sherlock takes her to London. (Also I think Mycroft knew about Mary’s past, but didn’t want to admit it to Sherlock. Look at his face in that conversation.)

So why is Mycroft so indifferent about the potential threat Ajay poses to Mary?