Taccone is the villain of our story. He's described as "a whole different kind of bad" (3.44). The implication is that yes, Kat's a thief, but Arturo Taccone, he might have killed people. This novel is all about different shadesof bad.
We're never quite sure exactly what Taccone has done, or will do in the future. We hear rumors of explosions and disappearances, and his flunkies have painful-looking brands on their hands. (Maybe they were just in Fight Club?) He's pretty relentless, too.
Taccone never once lets up on Kat and her dad; he won't stop until he gets his paintings back. And not only is the impetus for the entire plot of the novel, he also provides a good bit of internal conflict for Kat to chew on. She often ruminates on the whole "I'm a thief; he's a thief" conundrum.
So if Kat and Taccone are both thiefs, and Taccone is widely acknowledged as a bad person, then, then they must both be bad people, right? But Kat's doing her thieving thing to save her dad. Taccone's in this game to get his stolen paintings back.
We think Kat has what Taccone lacks: a conscience.