As a writer, I have to get into the nitty-gritty of everyday life. For some characters, that means they're thieves. They steal things, thinking it's okay, or for some other purpose that's more nefarious.
For the thieves that think it's okay, they're pushing the envelope of what's acceptable. 'Everyone else does it,' or 'it's just a few cents/dollars,' etc.
But stealing is stealing. Characters who are thieves have an interesting character flaw which is like gold in a writer's mind. They have a skewed sense of morality, that taking from others is okay.
When is stealing okay? That's the fine line the thief puts in his mind. It might be okay to say, steal a grape at the store, or to download a book free from a pirate site, but if you go to their home and take a spoon, for example, that's not okay. That spoon might be cheaper than a book, but that's not okay.
Thus, when writing about thieves, know they too, have limitations for their own morals. The old saying, 'there's honor among thieves,' also holds true. They look out for each other, protecting other thieves. However, making a thief change by the end of the book is no easy task. 'Once a thief always a thief' sticks in other character's minds. It's like the thief becomes untrustworthy, which is an almost impossible label to shed, even under extenuating circumstances for becoming a thief in the first place. 'I was stealing milk for my baby,' still makes that person untrustworthy.
I had to figure that out when writing 'Ruled a Suicide.' The main character, Ellie, was a homeless mother. She broke into a home for medicine for her daughter, also living on the streets. Once inside the home, she finds the homeowner, dead under the Christmas tree. Ellie calls the cops but can't tell who she is, for fear they'll know she'd broken in and she's homeless with a child. If anyone finds out she was stealing, she'll lose it all.
Have fun writing about thieves! They're interesting characters!