So I tell steam to uninstall the game. It claims to do so, but in the end it seems to leave it there. In particular, I have a folder

c:\program files\valve\steam\steamapps\MyUserName\dark messiah might and magic single player

It takes about a gig of space, even though I already deleted all the saved games. Does anyone know if it will mess up steam if I delete this folder?

you can safely delete the folder,
Steam will always remember you own the game, so you can reinstall it whenever you wish