This is due to a form of Obsessive Compulsive Disorder called "Pure O", which sounds like a drug Phillip K. Dick invented, but is actually a form of OCD that manifests as repetitive and extreme thoughts. They're most often violent or sexual in nature, but they can run the gamut from a religious person worrying that they're being blasphemous, to wondering if your partner is cheating on you. In my case, the intrusive thoughts were violent, although I guess a lot of people would also consider mass familial murder to be blasphemous.
It started when I was 15. I had been having disturbing thoughts for a couple of months and had been showing signs of anxiety for years, but for whatever reason, they hit me especially hard one day. I was watching an Ellen DeGeneres routine and I suddenly thought to myself, "Hey, what if you killed your entire family?"
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"None of them appreciate her understated, likable deadpan like you do."
And I couldn't shake that thought. It kept crashing into me, over and over, like I was in a murderous wave pool. I started thinking about how easy it would be to do it -- I knew there were knives in the house, and OCD has a way of making the absurd seem horrifically feasible. By the time Ellen started making a joke about petting her cat, I was fighting back an actual, powerful urge to stab everyone present. This may not be an entirely normal response to Ms. DeGeneres' comedy.