I read about half of his years ago, forgot about it, and just came back and binged the whole thing.
Subdued but touching ending. I was and am conflicted about the fact that Seiichi killed Shigeru, as he never seemed so totally psychotic before. But it did serve to show how dominating and destructive his mother was in his life. I appreciated the restraint in the closing act, without a fateful reunion with Yuiko or some other drama event. In particular it was nice to see that Seiichi and his father had a close relationship in the end, especially with their earlier distance and the scarcity of strong paternal relationships in manga/anime.
In the end, Seiichi had to come to terms with his mother and see her as she was, not as he imagined her to be. This was the underlying purpose of taking care of her: changing her diapers made it impossible to hold on to his idealized image. And this allowed him to finally release the pain he held and move on, knowing she was not a goddess or devil but just another person. To see him alone is sad, but to see him alive and able to enjoy each day is a victory. Even if he never realized an ideal life, together with Yuiko or someone else, few do. It's actually nice to see him have some satisfaction even in circumstances one might think of as terribly tragic.
If there's one message I took from this story, it's to know your parents. You are a product of the people who raised you in more ways than you (particularly if you're young) might realize. Reflecting on who they are can bring a lot of self-awareness.