Not butterflies, but moths. The pattern is that of a moth. The symbolism is probably that there are ugly things in the world, too.
Oh, thank you for the clarification! You are right, those are moths. However, the way I understood it, the moths were a representation of the guests in the room. You can see how they all wore masks and the twins could only see their scary eyes, eyes that turn into the pattern of the moth.
Then, the twins try to run but are devoured by those moths, which may be a metaphore of how they were “devoured” by the members of the cult there. They were surrounded everywhere, with the scary masks and eyes standing out, similar to moths.
This is so sad, omg. They hint you through the entire manga the horrible things they went through, but reading it directly is totally different. I wasn't prepared for this. Really, really, poor Ciel and Our!Ciel.
However, I liked the use of the butterflies as metaphore. I like how Yana tells you the story not with graphic drawings but more with subtle suggestions, and how despite that the manga does not lose its horror. The ending with the smiling statue is really strong.
I think what saddens me more of all this is how the twins, besides going through all that, get separated in the end. Their relationship is so strong, and Our!Ciel ends up all alone.