SWASHTOBER 8: Andrea Orsini
From Samuel Shellabarger's PRINCE OF FOXES, Orsini is a cynical courtier who's tasked with assassinating a prince and taking his place as a Borgias puppet (jackpot)! Only the target is a genuinely good fella who gives Orsini purpose greater than his own ambition.
There's a movie version of this from 1949, with Tyrone Power and Orson Welles, I haven't seen it, though, only read the novel.