You can. You can also sell derivatives on something you do own. If you get involved in human trafficking futures, you are still involved in human trafficking because your contacts could be taken to maturity, which means someone will have to provide the humans.

That's pretty much the definition of "complexity hides evil".

Index funds are a bit more direct since you're owning pieces of things you may not like instead of owning a contract that could be converted to something you don't like.