We're never going to solve the mystery of why Liz was there or who she was going to meet. Virtually nothing concrete is known about her life or the people she spent time with, beyond Michael Kidney. In this instance, it's pretty much irrelevant. We can either assuming that both murders that occurred on the night of the double event were not premeditated, but opportunistic, or that Liz's murder was not committed by the Ripper at all. I'm proposing that it may have been committed by someone working with the Ripper as a distraction while he killed somewhere else.