Dangit; I too thought this was titled 'Shriek of the Mutilated'.
Trying to remember the title right now.
Scenes that this includes:
The boyfriend, main character, makes a lone run for help at night while the others lock themselves in the cabin. He runs around this small community screaming for help to only be responded with lights being turned out and people otherwise ignoring him. He runs into a church where he hears singing and pleads for help, but the congregation continues to sing.
When he does get help from the sheriff, he is taken back to the cabin where, upon entering, they see the aftermath of the slaughter where everyone was killed, including the girlfriend with either an axe or large knife sticking out of her chest. The culprits responsible for this, one of which is dressed in a bigfoot type outfit from the neck down, happens to be walking out of an upstairs section, or attic, with a bag of loot.
Got to pick on Andrew for a moment: