From
here Fortunately, the library was an unlocked door, which was good, because Envy really didn't want to draw attention to himself by struggling to force it open. He slipped in, just as casually as anything, and adjusted to the dim light inside. As the thought came to him, he picked up one of the hardcover books from its shelf and slid it in the doorway, so that a crack of light shone in, and the automail girl would get the impression that somebody was inside; somebody friendly who was inviting her in.
Of course, this also meant other unwanted people might come to explore, but Envy brought plenty of syringes with him in the case that that happened, and he could do away with them easily enough; dispose of them with a solid blow to the head and be done with it. For the girl though; he had nothing on his mind so plain as just killing her.
He fingered one of the syringes and fantasized about how it would happen. She would come in, bright eyed and optimistic, calling out for her parents that were promised to be in this place. He would say something (he would have to practice the voice; he'd only heard those doctors speak once and hadn't memorized their voices carefully) or perhaps simply cough as if ill, and pique her interest. And he would take her quickly by ambush when she came near; stick her with the sedative and watch her crumble. She would experience much less pain after being sedated, but it was a sacrifice he was willing to make to prevent her struggling, since she had a fairly strong arm with blunt objects and while he normally wouldn't have cared, Envy was not as scratch resistant in this place as he normally was. Besides, if not in pain, she would scream in horror as he carefully, delicately broke each of her fingers, joint by joint, with loving precision. And he would let her go then, watch her break down as she realized her hands could no longer fix or heal. Watch Edward's fury grow. It was only a matter of time before the boy's anger made him do something stupid.
And then, he would flaunt it. Show That Man just how easily his favored son had succumbed to insanity and his own death by Envy's hands. Just as his father soon would.
Envy had come to this institute thinking he was in Hell. Now, he discovered, under his own control, it was becoming quite the opposite.