Lilly's choice not to steal seems right in step with her personality to me.
In Lilly's view, there is the group and the rest of the world. She is concerned with taking care of the group--which gets harder as it grows--but has never shown signs of actively trying to screw people over.
In episode 1, she didn't want to help Lee's group but wasn't trying to kick them out ince they arrived. She had the same reaction to the arrival of Ben.
On St Johns Dairy, Kenny suggested the possibility of taking over the farm if need be. She wanted to trade food and leave them be.
She's the only one that show concern for Lee and Mark. Kenny was busy plotting taking over the farm.
If you choose not to kill the brothers she seemed to give a look of approval.
I think Lilly is one of the most loyal and compassionate characters in the game. She puts the group above helping a stranger which makes her seem evil but it's hard choice she has to make, like Larry said.
Kenny will bring a stranger into the group and be pissed when there's no food or you feed Ben. What is that? And he's shown with Shawn, Larry, and Lee how opportunistic he is. He'll help or hurt you depending on how it serves him and his family.
By the end of episode 2 I was pro Lilly. She won me over.