I feel like a villain would only spare the life of a traitor in order to use them as a source of amusement (depending on their relationship and, if they are evil enough, their relationship may not matter). Possibly for some strategic purpose or to use as leverage depending on the traitor's relationship with other hero type characters. They may corrupt the traitor after learning more about them and use them for a purpose that the traitor would never normally participate in, possibly for a purpose that would horrify them (the traitor) and use this as a kind of torture as punishment for their betrayal. They may torture them so much there is no chance for any kind of recovery and send them back to their family, possibly their friends as an example, or just use them as an example to any others who would dare to oppose them. I'll leave what that example may look like up to your own imagination! It also depends on the genre of story, though. A fantasy villain may use the traitor as a form of amusement, like I said before, or they may just kill them. In science fiction, however, a villain may use a traitor as an experiment for some kind of advanced technology. There are very few times when a villain in any genre will allow a traitor to go free, no matter what genre you're talking about, unless they are doubting their status as a villain. Get creative with it, there are so many different reasons why a villain would let someone go, it all depends on how your story has progressed thus far.