Let Sleeping Dragons Lie
Theodore despises working in the library. The bookcases tower right up to the ceiling, a man-made forest full of things that one could never imagine, never even dream of. He has always been hungry for knowledge and knows that he should revel in having access to one of the best-stocked libraries in the wizarding world, but Theodore can't bring himself to linger there for a moment longer than is absolutely necessary.
The smell of ancient parchment is enough to make him feel physically sick. It doesn't matter to him what the contents of a particular book are: charms, hexes, it makes no difference. Every book there carries with it the smell of death that lingers around the old tomes in his father's library, books that Theodore had been forced to read as a young boy.
The dusty scent of old parchment is linked in Theodore's mind to things that should never be thought of, let alone written down. These books whisper such secrets in his ear as how to shatter minds, break bodies, and leave death in one's wake without any risk whatsoever of getting caught. He hates these books, hates how their words creep inside him and stir up the dragons that sleep in the darkest corners of his mind. He perceives these thoughts as dragons because that is exactly what they are like: they hanker after wealth, after fame, after blood. It is only when these dragon-thoughts are awakened by the scent of old parchment and the promise of ancient, powerful magic that Theodore cannot deny that he has become just like his father, just as power hungry, just as bloodthirsty, just as twisted.
This is why Theodore works anywhere other than in libraries. He curls up in remote corners of the dungeons, hides himself away in rooms that are adorned by cobwebs and damp, where it is too wet for the dust of ages to gather and the reek of old parchment to creep under his skin. His books are always new, the only notes in the margins written in his own, spidery hand. Sometimes, though, without realising what he has done, Theodore scrawls his thoughts onto the page for all to see: the parchment is ripped by a dragon's poisonous talons.
Theodore would rather not disturb these sleeping dragons; if they lie quiet then he can continue to pretend to himself that he only wants knowledge for his own sake, just like a dragon hoards gold for no reason other than its beauty; that he does not wish for the power to shatter minds and break bodies, to drive men to the depths of torment and madness without ever uttering a single word; that he is never, ever going to end up like his father.
Theodore became his father a long time ago.