I'm going to leave my cousin in an airbnb apartment on our last day of the trip
Why would you want to leave them there?
We have our reasons, but let's keep it between us, please
You shouldn't just ditch your cousin. That is wrong to just abandon someone like that.
You seriously need to relax. I'm sure he's fine.
Even though it may be fine, it's still extremely rude to do. If you wanted to leave him you should've communicated that with them so he wasn't caught off guard. It most likely would've made him a lot less upset with the.