Apart from leading to a dramatic conclusion, why did he keep Alice alive? He had rid himself of Lena and he was almost home and dry with Sophie so he didn't need Alice?
I thought initially it could be a clue to find out the truth to get her away but she didn't seem to want to get away from him so I just assumed as Henry Reed had tried to help her and got killed because of it.
I assume that he still had the need to have a girl held captive given his proclivities. Sophie seemed to have moved away from that so assume he still kept Alice because of it.