YA novel from the '80s about a boy who unalives his parents

I’m looking for a YA novel I read in the 1980s, probably published in the 1970s or 1980s. It’s set in the United States and the characters are in high school.

The narrator is a girl whose best friend is a boy who also lives in her neighborhood (they’re neighbors as well as best friends). One day he murders both of his parents, and I think he then kills himself too. He deliberately does it the day before his mother’s birthday, and there’s a line or idea that by killing her then he “takes a year off her life,” so she dies at 40 instead of turning 41 (or something like that) because she was very vain about her age.

I have a strong memory that the mother was found dead at the kitchen sink wearing rubber dishwashing gloves, washing dishes when she was killed. She was very vain about her hands as well as her looks.

The town and school are full of gossip and scandal afterward. The girl narrator really wants to believe that someone else killed them, but as the story goes on it becomes more and more clear that her friend really did it.

It’s realistic contemporary YA, not horror or fantasy. I think there might have been something about him being adopted or having issues about being adopted, but I’m not sure—that might be me confusing it with a real-life case from where I grew up.

Does this ring a bell for anyone?