I'd say it's inappropriate, but I don't think it's "wildly" inappropriate, if that makes sense. Is he suggesting that you are disrespecting her memory with some of your decorations? Imo, it's Halloween and that allows for decorations like that...

I mean, we get people walking around in costumes that are insensitive in a bunch of ways and any person can bed insulted for any number of reasons... say someone dressed up as a zombie wearing a military uniform, people could get insulted because it's disrespecting the armed forces.

I think his opinion of your decorations simply shows that too many people try and look for things to make a fuss about... You said he didn't even know the neighbor who was murdered, which tells me he is the type that is just trying to stir up arguments and trouble. I would say to just ignore him, because he's probably doing this for attention and giving him that will only reinforce the behavior.