A Cure for Conspicuousness
Oh, Milner. You were so close to getting away with it. You finally convinced the Utopia fans who'd just spent six episodes unravelling the Corvadt (Illuminati) "vaccine" conspiracy that you were on their side after all. Did you have to shoot a hapless henchman to do it? Sure. But you know what they say about omelets and eggs, right? Plus he knew the risks. Or he didn't. But since one of the principles underpinning the whole plan is to reduce the human population, you might as well do some herd-thinning yourself.
Anyway: you had won the trust not just the Utopia forum pals but Jessica, the most paranoid member (justly so, but still). You got her to MI-5. You had her on the verge of handing over the manuscript and co-operating. But you got cocky.
Granted, by the end of the episode we learned that the vaccines in the warehouse were a very expensive diversion and that the manuscript itself was just a red herring, and that the main point of the exercise was to secure Jessica and her special blood. But of course she knew what the rock meant; she'd had one herself once. And whatever horrible things you will undoubtedly do to her next season might have been easier if you could have kept up the elaborate ruse -- and you could have, if she hadn't figured out your real affiliation and intentions and gotten so spooked. But you didn't think to tidy up your office before she got there.