Joined the bandits? If you leave her when she kills Carly/Doug or if you keep her and she escapes with the RV. Since she has NO ONE left. Larry died, she betrays the group and the group is not accepting of her. Heck, she's desperate.
I could see her joining the bandits since she has nothing left and she could see joining the bandits as a chance as survival. Knowing Lilly, she does what she can to survive. Since she is loopy and unstable after her witnessing her dads gruesome death, she'd fit right in with the bandits.