“literarymagpie replied to your post: ...”
Ah, question answered. Seems to me like the bit that takes most explaining isn't why it was okay to kill his brother (because there are umpteen bits of backstory one could add to deal with that) but why it was okay to marry Gertrude immediately afterwards – and to displace Hamlet in the succession, if that's how the succession laws work in the setting. I guess it would be to strengthen his position, but it seems like that makes it necessary for Hamlet to be a threat?
Well, for one thing, I don't want this to be a good novel - I don't think I've ever mentioned the one I read in which the Sheriff of Nottingham was a Good Man who was raising taxes in the shire because his predecessor had left everything a mess & he needed the money not for himself but to make the castle run better. I want it to be along the lines of that.
That being said, I think the putative author of this novel, who is absolutely not me, would have to go back to what the jacket would call "the real history behind Shakespeare's Hamlet." The story is from Saxo Grammaticus, and the oldest version is 12th century. I think Scandinavia had elected kings then, so it would be up to the Council, and also I can't tell from Saxo, but I think he might not have been at age at the time of his father's murder.
Admittedly, Saxo says "Then he took the wife of the brother he had butchered, capping unnatural murder with incest," so it's really revisionist, but oh well. You could take Fenge/Claudius' word for it when he says "Gerutha, said he, though so gentle that she would do no man the slightest hurt, had been visited with her husband's extremest hate; and it was all to save her that he had slain his brother; for he thought it shameful that a lady so meek and unrancorous should suffer the heavy disdain of her husband." So he's saving Gertrude from a terrible marriage and is her true love.
Again: this is not a good novel. It is a novel I would make fun of. But I want it.
The legend was taken as the basis of a 1994 film by Gabriel Axel, Prince of Jutland (also known as Royal Deceit), with Gabriel Byrne as Fenge, Helen Mirren as Geruth and Christian Bale as Amled.
I want to see this movie. If you have seen it, by some strange chance, I don't want to hear that it's bad.