If you've ever had the pleasure of dealing with an abattoir worker, the emotional strain is telling. Spending day after miserable day slaughtering confused, scared, captive animals until you're covered head to toe in their blood is... not good for your mental health.
The Texas Chainsaw Massacre often gets joked about because this "based on a true story" wasn't in Texas and didn't involve a chainsaw and wasn't a massacre. But what it did get right was how Ed Gein, the Plainfield Butcher, had his mind warped by decades of raising and killing farm animals for a living.