In my main game, the one I always play first with my first instincts, I have Doug instead of Carley. When Lilly shoots at Ben and Doug pushes him out of the way, I found it pretty easy to reason it and say that she just finally snapped, so I let her back on the RV until we could figure out what to do with her.
Then I played my second save file, where I try different things to see what happens. In this one I have Carley. This time, when Lilly deliberately shot and killed her and didn't seem to be sorry about it like she was with Doug, it was a whole different ball game. I left her behind and didn't think twice about it. Anyone else experience anything similar?