Criminally Insane (1975) 
Obese Ethel gets released from a psychiatric hospital under the care of her Grandmother. The doctors fear for her health and warn her that she must stop eating so much and lose some weight. However, Ethel doesn't want to stop eating...
In fact, she wants to eat more and more to the extent her grandmother locks all the food away in a cupboard with the intent of forcing Ethel to diet. What poor Grandma doesn't realise is that you should never keep a psychotic fat person from their food, and she soon gets a knife in her back for her troubles. Now Ethel has control of the food and starts to eat all she can and kill all who get in her way.
Criminally Insane is a fairly short low budget almost 'slasher' film with OTT ketchup-red bloody kills as Ethel hacks and carves her way through all barriers to reach the fridge.
Short and sweet, Criminally Insane was an entertaining ride with a simple no-nonsense story which didn't try and bite off more than it could chew; unlike Ethel, of course.