A man so wrapped in secrecy that his shadow doesn’t have a high enough security clearance to stick to his heels. - Charles Stross

Time travel destabilizes history.
History is a child of contingency; so many events depend on critical misunderstandings or transient encounters that even the apocryphal butterfly’s wing is apt to stir up a storm in short order.

“Prosthetic Morality Enforcement. The idea is that by analogy, if a part of your body is deficient or missing, you can use a prosthetic limb or artificial organ. Well, our ability to make moral judgements is hard-wired, but it’s been so far outrun by the demands of complex civilization that it can’t keep up. For example...have you ever wondered why discussions in chat rooms or instant messaging turn nasty so easily? Or wander off topic? It’s because the behavioural cues we use to trigger socially acceptable responses aren’t there in a non-face-to-face environment. If you can’t see the other primate, your ethical reasoning is impaired because you can’t build a complete mental image of them—a cognitive frame. It’s why identity theft and online fraud are such a problem: There’s no inhibition against robbery if the victim is faceless. So we need some kind of prosthetic framework to restore our ability to interact with people on the net as if they’re human beings we’re dealing with in person.

He’s found a teapot and milk and is brewing up, because that’s what the English do when they’ve just broken out of military prison one jump ahead of murderous assassins from an alien death god cult, then survived being shot at by a main battle tank.