Gah, I'm still flip-flopping between Henry and Madison. Neither are particularly good as the killer since they both require fairly dumb rationales to make work, but I honestly cannot see any one else even remotely as a candidate.
-I think Henry is definitely Wakefield's son.
-Being Wakefield's son doesn't necessarily mean he's the killer.
-I don't think Madison necessarily has to die - unless she's the killer of course.
-Abby will definitely survive.
-If Henry doesn't survive, Trish just might.
-Henry and Trish will not both survive.
-If Henry is the killer, he will kill Trish.
Okay I got another theory, even dumber, which better not come true.

Shay is being driven insaaaane with rage at her cheating hubby and taking her rage out on the whole wedding party and helping Wakefield slaughter everyone. Both Henry and Madison are innocent.
Or, hmm. How do we know Wakefield and the other killer have ever been in cahoots? It might be interesting if it were a mere coincidence that there are two killers on the loose at once? Maybe I should stop looking for a connection between Wakefield and the other killer.