"Miss Bishop did willfully... um... steal personal property." (1.24)
Kat clearly has an ambiguous sense of morality; she's not really upset that she's been accused of stealing the Headmaster's car. She's upset that they think she'd actually get caught if she did do it. Haha.
[Kat] thought about crime, as she so often had in her fifteen years—ever since the day her father had told her he'd buy her ice cream if she would scream, and keep screaming until one of the guards outside the Tower of London left his post to see what was wrong. (3.62)
Thieves aren't supposed to want too much—which is ironic, but true. Never live anyplace you can't walk away from. Never own anything you can't leave behind. (4.13)
This seems to be part of the thief's complicated moral code. You'd think that a person whose job is stealing things would be a little more materialistic. But if you're always trying to be two steps ahead of the cops, you probably shouldn't get too attached to anything.
"It takes a thief to catch a thief." (14.75)
In other words, it takes someone with the same moral code (or lack of one) to get into the mindset of another thief. Does this mean author Ally Carter has pulled off a few heists in her day? Probably not. But it's fun to dream.
"Somebody's playing games! [...] Somebody's having fun! And he doesn't care that other people are going to get hurt because of it." (15.64)
As a thief, Kat doesn't mind when other people steal things. She does mind, however, if other people get hurt in the process. Especially when those people are a part of her family.
"We're not stealing from the Henley. We're stealing at the Henley." (25.20)
Kat makes an interesting ethical distinction here, which, in turn, distinguishes her from, say, Arturo Taccone. Kat and her crew are only stealing what has already been stolen. They're not just taking art from the museum for their own profit or use.
Every good thief knows that the only job that matters is the next job. (26.7)
Thieves have pretty good work ethics, too, if we do say so ourselves. Like a model, freelance writer, or daytime soap star, their eyes are always on the next job.
[Arturo Taccone] was still a common criminal. But then again, Kat realized, so was she. (29.1-29.2)
What separates Kat from Arturo Taccone, morally speaking? How different are they, really?
Kat was thinking about Abiram Stein, whispering even if only for herself, "I know someone who has been looking for this." (32.60)
Even in the middle of an art heist, Kat has a strong moral center. She recognizes Two Boys Running Through a Field of Haystacks and instantly thinks of returning it to Mr. Stein, the man who lost it in World War II.
"Rest assured, the Angel is safe and she is happy. The enclosed belongs to you. It is time that it, too, returned to its family." (37.59)
Here, Visily Romani shows that even he has an ethical code. Perhaps his plan all along was to get these paintings returned to their rightful owners. We're just not sure why he'd do that, or why he sends Kat this painting at the end. It's sequel time, Shmoopers.