Copyright and theft

I do realize that illegal things can be sinful (some venial), but I started to recently wonder if fan edits of comics/games/movies/etc would be considered theft because it falls under copyright.

This might sound dumb but you're basically making short edits of things that aren't yours, but I wouldn't be profiting from it either even if I were to post them. And if I were asked to take it down, I would. (But it hasn't happened).

Anyways, some input would be nice, because I do not wish to be sinning even if venially, though editing is like a great hobby of mine.