Abrupt conclusion to volume 4 of the Patrick Melrose cycle

The 4th volume of Edward St. Aubyn's Patrick Melrose cycle, Mother's Milk, ends with a whimper - as the last 40 pages or so concern Patrick's efforts to fulfill his mother's final wishes and help her kill herself as she lies nearly immobile and incoherent in a nursing facility; he's of course deeply ambivalent about doing so, concerned about the legal issues and about the obvious dichotomy between filial loyalty and matricide - moreover we know she's been a horrible mother to him and has also essentially disinherited him and part of him would really like to kill her, just as he wished his (even more) evil father dead and gone long before. Honestly, however, this is one of the weaker sections of the series as it all plays out within Patrick's mind - it's a narrative element that would be stronger and more touching in another context, say, someone faced w/ decision of helping the suicide of someone whom they truly love. Finally (spoiler here, if you care), his mother, Eleanor, gives them the message: do nothing, bringing this volume to an abrupt and unsatisfactory conclusion. So he doesn't carry it out after all; why exactly she changed her mind, other than perhaps to continue this drama into the 5th and final volume?, is completely unclear and unexamined, like so much else in this series - such as, how or why did Patrick and Mary ever meet and get married anyway? It feels in these pages as if St. Aubyn is just working through the plot - without the sharp wit and keen observations of earlier passages: even when I hated the characters at the most, which was often, I have found myself underlining passages and making check marks in the margins (no I'm not reading this on my iPad). Having come this far, I will read At Last later (it's not part of the edition that I have just finished), but it's a dark journey that has halted abruptly and I hope the final volume brings this cycle to a true, earned conclusion or closure: I know St. Aubyn, like most writers, emulates Proust and he can arrive at the level of grandeur and pathos that Proust "captured" in Time Regained.