Macabre(1980) a woman cheats on her husband so while she's with her lover her older child murders the younger one by drowning him in the bathtub. While racing to the scene mom and her lover are in a horrific car crash; lover is decapitated. Mom goes into the nuthouse for a year when she gets out she keeps dead lovers head frozen in the freezer periodically having sex with it.
This sounds amazing. I must see this movie.
Aaaand ... added to Netflix queue. This forum is great.