To deter away from the actual crime. Isn't that what framing and deception are usually about in these cases?

As it is, there are way more Halloween-relevant clues in this case. For all we know, some of them might not even be related and are placed here to fit the whole theme of the mansion itself, rather than the actual murder.

Maybe one of our suspects is enthusiastic about the season and felt the need to play along. [ A pause, then a smirk. ] If that's the case, you can cross Spain off the list.