I'm not sure there's a whole lot of novels where the protagonist is the god-killer from the first page, but the ones that explore that journey feel so distinct depending on the genre. 'The Poppy War' trilogy by R.F. Kuang is brutal in this regard—Rin's relationship with the Phoenix god isn't about slaying it so much as becoming its vessel and then wrestling with that power on a geopolitical scale. It's less about a clean kill and more about a horrifying symbiosis. The divine violence is intertwined with real historical atrocity, which makes it land heavier than a typical fantasy romp.
Then you have stuff like 'Godkiller' by Hannah Kaner, which takes a more traditional quest approach but with a tired, worn-out protagonist who's seen too much of it. It feels grittier, like killing gods is a messy trade. I tend to prefer the stories where deicide has consequences—like the world breaking down, or the killer becoming something they hate. The 'Mistborn' era one finale is a famous example of that, but saying more would be a spoiler. Honestly, a lot of 'best of' lists just focus on the power fantasy angle, but the more interesting books ask what you're supposed to build after you've torn heaven down.