I'm thinking of writing spoilers from endgame in random places people don't expect to see them so they can't avoid it; I haven't actually seen it and I'm just making it all up.
That's rude. Why would you make up fake spoilers?
Because it's fun! Plus, they aren't real spoilers so I'm not actually ruining the movie.
It is really rude to try and ruin someone's movie experience with fake spoilers. People will assume those spoilers are true and will be thinking about them the entire time they are watching the film. People get joy out of watching movies and you are trying to ruin it.
You can ruin things you don't like.
Intentionally trying to ruin someone else's experience with something just because you don't like it is selfish and rude. It's perfectly fine not to like a movie, but you should let others enjoy it. Instead of dwelling on things you don't like, focus on things you do like.