There does seem to be a general pattern in all the books of ignoring the consequences of letting your enemies run off and forgetting about them. Garland and Grita both come to mind. You would think that someone with experience would see the pattern.
(Excepting perhaps that this is a thematic element, and thus invisible to the characters within the story.)
Villains who have been allowed to go free, then returned for vengeance:
Villains who went free, but have not (yet) returned for vengeance:
- Tri'nagore (That we know of... but see Tri'nagore (Speculation))
- Herth and Bajinok
- Sorceress in Green
- Sethra the Younger
- Seodra (died in prison so not likely, unless it was a plot with a long incubation period...)
Will we see more from any of these?
Let's look at the pattern of the Paarfiad as it now stands:
In The Phoenix Guards, Garland is a minor villain, manipulated by larger players. At the end, he runs off, and is not captured. Illista is a more major villain, and she is exiled. All of the other "villains" are accounted for (Lytra - beheaded later on; Seodra dies in prison).
But wait - there's an exception in Allistar. He's only a minor player, but he is one of the conspirators. He's not accounted for, yet never shows up again either. He's not mentioned as a surviving Phoenix by the Gods, but the Gods may be clueless.
Another exception: The Jhereg that Seodra was working with; Fayaavik. I don't think he shows up again later either.
In Five Hundred Years After, Grita is a minor villain (although more major than her father was in the previous book). At the end, she runs off and is not captured. All of the other villains are accounted for (Garland - killed by Khaavren; the various Jhereg who he's working with are either killed when they make assassination attempts, or in Adron's Disaster).
Hm. Is Mario a villain? Well, he is a murderer, no two ways about it, and quite possibly contributed to the Disaster occurring. Paarfi indicates that he was saved by Sethra -- but he's not accounted for, by any means.
And come to think of it, Aliera e'Kieron was acting sort-of villainously in that one.
OH! And Adron e'Kieron, whom we thought was killed in the Disaster along with everyone else in Dragaera City, was revealed to us to be somehow still in some sense aware somewhere inside the Lesser Sea of Chaos. So he's less well accounted for than we might think.
In The Viscount of Adrilankha, the only ones who definitely were not permanently killed were Tri'nagore and Loraan, and Loraan we know died in Vlad's era. Everyone else is accounted for by the end of that volume (Kâna and Habil - executed; Illista - crunched by Tazendra; Grita - killed by Piro; Tsanaali - spiked by Pel).
Speculation: Will a future volume by Paarfi involve Tri'nagore, Mario, Aliera, Adron e'Kieron, and maybe Allistar?