I am open to the possibility that Martha may have been the first victim of Jack.
The issue with Martha was that she was stabbed 39 times in her throat and lower abdomen. While this lacks the typical Modus Operandi typical of the other Ripper murders, we know that Serial Killers evolve over time, "perfecting " their craft so to speak,this would likely be true. I have mentioned before that Jack may have been a picquerist, and that he may have been impotent. Perhaps the Ripper may have picked up Martha with the intentions of having sex, but was mocked when he could not perform, and in a rage he murdered poor Martha, the murder provided him with the release he needed.
Two books to reference is Patrica Cornwells "Portrait of a killer: Jack the Ripper case closed," and Peter Vronski's "Serial Killers: the method of madness and monsters."
Vronski's book is a particularly interesting read, in it he covers interviews with apprehended Serial Killers, of which they mention that the first murder is often unplanned, but always the most significant.