MS's will just highlights why Jeremy would commit the massacre.
Jeremy was a very slippery character, so I wouldn't put it past him to have found out what was in MS's will. He knew about Neville's will & that June was planning to amend her will. If he did know he was not included, it would just increase his desire to commit the massacre.
If he had not seen the will, he would assume the worst. If MS had a good relationship with June & her sister, there is no reason why they should be bypassed for Jeremy. Increasing his desire to commit the massacre. It is doubtful he was very close to MS, see previous post.
If MS was in good shape, she may have lived several more years. Increasing his desire to commit the massacre.
With everyone removed, Jeremy would inherit all of MS's estate in the future. June & Neville not having the chance to spend it. Increasing his desire to commit the massacre.