Non-linear question (spoilers)
After the ending I'm not sure if Miriam did the thing. If she did, I am confused about when she did it.
At the final scene of the party, we don't see Dylan, but nobody is concerned about him being missing so I assume he's present.
And, Miriam has long hair so either everything happens after this party (and her tearful final expression is showing us that she is deciding whst she will do after this) or, did everything happen in her imagination?
I'm leaning toward the latter because, how did she become such an expert in murdered body disposal? I think the trauma made her live an alternate scenario in her mind where she let her vengeance run free.