System for really large group one-shot?

Every year, my youth group pulls an all-nighter.

Given that we do these in church and at night, with a group of 15-20 people, I think this would be the perfect environment to do a group Murder Mystery campaign, where I’d introduce some sort of moral debate – for example, have an Angels & Demons sort of plot, with several religious members being killed, and, in your search for the killer, you have to look into their motivations and debate the sins of the church itself.

I have some experience with D&D but I really don’t know of any other formats, much less ones that would work with a group this big (in terms of logistics, it doesn’t make sense for people to have to create very complex characters or work with a lot of rules). I also like the idea of a LARP, because it would make more sense for the people who’ve never played any RPGs.

Does anyone have an idea of where I could start looking for something like this?