Not Killing a Villain directly has many different roles in writing. While it can be used for virtue signalling and to keep a characters purity it also can be used at the heart of a conflict. A Large part of the last season of Avatar the Last Air Bender dealt with Aangs struggles to stick to his values of not killing when everyone (Zuko, Sokka, even Ozai himself) says the only way to Stop Ozai is to to kill him. Naturally Aang finds another karmic punishment for the Fire Lord (removing the man’s ability to bend and letting him rot in his own palaces prison). Similarly the main conflict Between batman and Jason Todd is Bat’s own refusal to kill, and he has never been truly able to come up with a good reason for why he let’s madmen like the Joker live. Other Times a VIllain is spared because they didn’t really prove to be super evil, as with Vulture in Spider-Man Homecoming, definately evil, but at the end of the Day he was just trying to Rob Planes and Sell illegal weaponry, so probably didn’t deserve to die.
Other Times it’s for narrative cohesion. Yes, Optimis Prime should probably just go for the Kill-Shot against Megatron, but then who would the shows main villain even be? Starscream? Or perhaps it’s to give the villain a truly Karmic or ironic death, such as in bioshock when Atlas get’s stabbed to death by a plethora of Little Sisters or when Dr. Suchong get’s impaled by a Big Daddy. Sometimes Living is punishment enough, as with Ozai in the example above (Lost his powers and empire, and now lives at the mercy of his abused son)
I could see them going Karmic or Ironic fates for Sylvanas, some of which i don’t think she has to die Depending on what the punishment actually is) but it entirely is based off how it is written. And I don’t trust blizzard enoguh to write it well.