What is the name of this movie I'm thinking of?

I started to watch a movie maybe a year ago where there is this normal guy -- he's like middle aged with a nice house and kids who are already grown or maybe in college and I keep picturing him as Kevin Spacey --but at night he goes out to a shed in the woods that is like an arsenal with a giant furnace and gets weapons and goes and kills random people, comes home and destroys the evidence, and continues his seemingly normal life. I don't know what happens after that but the first people he kills are this couple having sex. Please help me remember the name of this film.