Interesting point, Col Klink.

In a lot of media, the good guy killing off the bad guy is the big, exciting struggle at the end story — for instance, IIRC, most Disney animated features are like this. Thinking about Narnia, though, Lewis often seems to give a lot more attention to moments of joy rather than the comeuppance of darkness — the statues coming back to life in LWW, the Romp in PC, et cetera. Even in the battles of wits and steel with the Green Witch in SC, it's Puddleglum's speech that stands out as the Big Moment, not the struggle with the serpent afterwards. I think almost everyone thinks of Puddleglum as the hero in that chapter, whereas actually killing the serpent is a messy, chaotic team effort instead of an epic battle.

I guess I would say that in his books, Lewis did not seem primarily focused with the defeat of evil, and his main characters having the glory of its defeat, but with going further up and further in, seeking Joy. Defeating evil was a necessary step in doing that, but it wasn't the point of the story, if that makes sense — and therefore the identity of who slayed the villains wasn't as big of a deal. A J.R.R. Tolkien quote comes to mind: "I do not love the bright sword for its sharpness, nor the arrow for its swiftness, nor the warrior for his glory. I love only that which they defend." I hope any future writers of Narnia adaptations will appreciate this tendency of Lewis's storytelling!

@col-klink This is very interesting. I haven't thought of this one.

It is true that Aslan killed the White Witch (and he actually meant it). People have speculated that the White Witch and the Lady of the Green Kirtle are the same person, which they are not. I'm sure there are lot of Northern Witches besides those two, but not necessarily the same one.

Glozelle and Sospesian killed Miraz because they weren't all that loyal to him (like the hyenas weren't all that loyal to Scar) instead of Caspian or Peter or Edmund.

There's no clear cut villain in Voyage of the Dawn Treader (unless you count Governor Gumpas and Pug as villains, but I think they were just minor bad guys). After Caspian and his crew sailed further East after the Lone Islands, we aren't told what became of Governor Gumpas or Pug.

Prince Rilian, Puddleglum, and Eustace all kill Lady of the Green Kirtle when she's a snake. He knew at once it was the same snake that had killed his mother over ten years before.

We aren't told of Prince Rabadash's fate after being turned into a donkey then being turned back into a human. We are told that he eventually became Tisroc (equivalent to Pharaoh in Egypt or Sultan in Arabia or Czar in Russia) after his father, and he was known as "Rabadash the Peacemaker." Other than that, we're aren't told.

There's not a villain death in The Magician's Nephew. We are told however that Uncle Andrew does become a better person (unlike Miraz in Prince Caspian).

Ginger goes into the Stable, and we're told later it was Tash he saw in there. It freaked him out, and runs out of the stable. He is struck into a dumb cat; he can't talk anymore. He runs up in a tree, and is not seen again (I think it would be safe to say that Ginger was among the animals who turned left and disappeared into the shadows). Tirian hurls Shift into the stable, and we're told later that Tash ate him (it's after the fact, but we still know it happened). Tirian pulls Rishda into the stable with him, and Tash shows up, taking Rishda. Peter banishes Tash, and he vanishes, taking Rishda with him. After that, we aren't told what have become of Rishda (possibly eaten by Tash like Shift... or worse.. I don't want to think about it).

So to your point, they are villains deaths we are told. There are some villains we aren't told what have become of them. It's leaving us to our own speculations.