villain stalking each other" illustration. Look at Flandry's face.)
As a rule, recurring villains are never killed, because they are recurring characters, and never reformed, because they must return as villains. There are exceptions.
In Poul Anderson's Dominic Flandry series, when Aycharaych's motivation to work for Merseia, therefore against Terra, had been removed, he might have reformed but unfortunately he never returned. Also, he would have had to overcome his aesthetic pleasure in the manipulation of intelligent beings and the fomentation of strife.
IIRC, SM Stirling's Rudi Mackenzie spared his antagonist, Graber. Will anything come of this? Might Father Ignatius be able to exorcise whatever it is that possesses leading Cutters? I have a poor track record for anticipating authors but I wonder whether anything might come of this line of enquiry. In another timeline, I imagine that Stirling's Count Ignatieff was irredeemable.