It's entirely too amusing, that a part of Mamoru still believes in fairytales. It is also very, very annoying.
Nagi might not be a telepath, but he knows Mamoru, knows him more than he lets on. He knows that Mamoru still believes that what they are doing is for the greater good, still thinks that they're doing something that needs to be done, still believes in the knight riding the white horse over the sunset.
Mamoru doesn't say so, of course; he pretends that he killed Omi completely, asks his old friends to stop calling him that. They're too blind to notice the fact that little Omi is just playing pretend so well that he has fooled even himself.
That changes, Nagi knows, when Mamoru looks at Weiss, knows it at the look Mamoru gets when Fujiyima walks away and he snorts as he moves towards Mamoru and presses against him, pushing his tongue inside his mouth, feeling how Mamoru shivers before he kisses him back, before he moves his hands to touch him.
There are some things that only change if you're willing to change them yourself, after all, and Mamoru is not strong enough yet to do so.