Satan, that ever elusive talking serpent of old, is just a metaphor for a despicable human archetype, a sadistic malignant narcissist. Killing Satan is not for God to do. Killing a malignant narcissist is not really helpful, even if satisfying. Casting them out of their lofty positions and publicly embarrassing them deflating their ego making them live in shame as the lowest of all lowlifes instead of being celebrated as beautiful, stars, wealthy geniuses (with really good hair), is work for humans to do, and by far a fate worse than death for every egocentric cruel petty tyrant. Think! "Thy will be done, on earth as it is in heaven" To understand what that means see Gen 3:14
Not paying homage to a devil, putting Satan in its place as the lowest of lowlifes is the only way to rid the world from the deadly effects of being bitten and dying from the poison of a talking serpent, a descendant of Satan.
So the question really isn't why doesn't God kill Satan, the question is why humans haven't done that yet, WTF, whenever and wherever he raises his ugly head. When Satan asks you do do something. Just say no. lol. its easy.
A devil without his minions, er, possessions, is like an unarmed soldier running bigly into battle completely naked.
A joke.