Hitchcock was famously frustrated by the change, but his hands were tied by the Motion Picture Production Code (the Hays Code). The code strictly prohibited a murderer from getting away with their crime without facing legal "retribution."
Here is how Hitchcock and the production team felt about the shift
A "Necessary Evil": Hitchcock knew that to get the film made in Hollywood, Maxim couldn't be a cold-blooded killer. In the Daphne du Maurier novel, Maxim shoots Rebecca in the heart. In the 1940 film version, it is changed to an accident where she falls and hits her head during an argument.
Weakening the Narrator's Arc: You hit on exactly what Hitchcock lamented. In the book, the narrator’s "loss of innocence" is much darker; she becomes an accomplice to murder and is thrilled by it because it means Maxim didn't love Rebecca. Hitchcock felt the "accident" softened the psychological impact of her choosing a killer over her own morality.
Producer Clashes: Producer David O. Selznick was obsessive about staying "faithful" to the book, but even he couldn't bypass the censors. They compromised by making the fire the moral "punishment" for Maxim's cover-up, even if the death itself was unintentional.
Hitchcock’s View on Maxim: Hitchcock leaned into the "jerk" qualities you noticed to compensate for the lost legal guilt. He directed Laurence Olivier to be cold and irritable, ensuring the audience still felt that "danger" and unease around him, even if he wasn't technically a murderer.
By making it an accident, the film shifts from a moral thriller about a woman loving a killer to a melodrama about a man haunted by a secret.

The Motion Picture Production Code rules as enforced in those days would not permit Maxim to "get away" with murder as happens in the book.
Now, one could argue that Rebecca's deliberate goading of Maxim into killing her."suicide-by-goaded-and-infuriated-husband", as it were.was at least a partial exoneration of Maxim's guilt.
Well, maybe if the movie was made today. But not in 1940. So, Maxim had either to be punished for murder or be shown to be innocent of the killing. I think Hitchcock made the right choice given the options available at the time.