Madhouse (1981)

Title: Madhouse

Alternate Titles: There Was a Little Girl ( And When She Was Bad, Party des Schreckens) 

Release Date: 1981

Tagline: "Many people visit ... No one ever leaves."

Plot
Julia (Trish Everly, in her only film role), a young teacher for deaf children living in Savannah, Georgia. Julia has horrid memories of her childhood, which was scarred by her sadistic twin sister Mary (Allison Biggers). At the urging of her uncle, Father James (Dennis Robertson), Julia visits Mary, suffering from a severe skin disease, in a mental institution. The meet does not go well, and Mary vows to make Julia "suffer as she had suffered". As their mutual birthday approaches, several of Julia's friends and neighbors begin to die gruesome deaths, some of which committed by a mysterious Rottweiler dog that has some sort of connection to Mary. But is Mary really the killer? - (from wiki)

Review
Didja ever watch a movie and afterward get the uncontrollable urge to track down the people who made it and, when they answered their door, to just punch 'em right in the head?

Just asking.

ME: Your Honor, I present the following evidence in my defense.

Exhibit A: Exploiting a genetic condition and equating it with mental illness.

Exhibit B: Characters who do stupid time-wasting things while waiting to get killed.

Exhibit C: Characters who talk to each other as if they were talking to 2-year-olds.

Exhibit D: Killers who sing nursery rhymes while killing.

Exhibit E:

JUDGE: (Bangs his gavel) That's enough. Sentenced to time served. You are free to go.

handbledzoin from United States 17 May 2011   (review from imdb)