A is for Alibi. Open spoilers. Is a major plot point not explained?

Inspired by the thread on mystery book solutions. I’ve been reading the Alphabet series books by Sue Grafton, and the first book in the series has a major twist that I still don’t understand. Towards the end, Kinsey Millhone solves the initial murder of Laurence Fife. It turns out to be his ex-wife, Gwen. There was a second victim, who was killed in a similar manner, but by a different killer. The second killer, Charlie Scorsoni, decided to kill the first killer, Gwen Fife, the very night she is confronted about being the murderer and confessing to having killed Laurence. What I still don’t understand is why Scorsoni thought he needed to murder her. The way the story was going up to that point, it’s likely he would have gotten away with the two murders he had already committed had he not decided to kill Gwen as well. Gwen had no knowledge of Scorsoni’s involvement in the second murder and her giving a full confession to the police wouldn’t have helped in catching him. The question is why did Scorsoni feel the need to kill her?

Just bumping this in hopes someone can help Flik here. I love the series, but A was a lonnng time ago. And I soon discovered that it was the characters I loved, not the case itself.

The audiobooks are wonderful, too, read by Judy Kaye. She makes a great Kinsey Millhone.

Ha, I just googled and one of the first hits is “Sue Grafton Alphabet Books In Order”. Yeah, that’s a toughie…
(though I read E, then T, then A, I, O, N, S, H…)