My hostile airbnb host left out his key so I stole it and copied it and now I still have it.
What do you plan to do with a duplicate key?
Time to double my payment and start stealing
By stealing and harming an owner, you immediately lower your standards and become untrustworthy, which will cause you to lose your reputation on Airbnb. Just throw away the key and move on from the hostile host that you dealt with after leaving an honest review.
I just don't see why I should have to give it back, that's all.
You're effectively being a thief and society looks down on people who steal for their own gain. Do you want to be disliked by others? If not, then give the key back.