Lilly was paranoid and selfish when she killed Carley/Doug... she KILLED them, she shot Carley saying that it was the best for the group... she just hated Carley since the very begining. However... Kenny (and lee if you sided with him in the Meat Locker) gave Lilly a trauma, i mean, she saw how his father's head was smashed right in front of her, he was all what she had got left, and everyone in the group (but lee, if you sided with her in the Meat Locker) against her. Lastly, Ben could have told the group about the bandits, he didn't make the deal with the bandits to save the group, but to save his (already dead) friend. Ben is just a coward, i mean, Clementine can be braver than him in some situations. If Ben told us about the bandits earlier, everything could have turned out different; Carley/Doug wouldnt have been shot, Duck wouldn't have been bitten, Katjaa wouldn't have killed herself, and Lilly wouldn't have left the group.
~Goodbye, she quietly says~