I don't see how this is a bad thing.
In one of my isolation fortresses that went hilariously awry, I actually fed my fortress by butchering diplomats.
I'd open the main gate and allow the diplomats and traders in. Then seal off the gates. They will eventually go insane or berserk, get struck down, and the corpses hauled off to the butcher's workshop.
The entire time dwarves were going insane and throwing tantrums. Refuse stockpiles were overflowing. There were severed limbs and multiated corpses scattered all around the fortress. The entire fortress was a huge purple cloud of miasma. They could not produce enough food to feed everyone or to brew booze, as I had embarked on a terrifying glacier. I was under endless siege from above and had to keep the drawbridge shut to protect my dwarves from the endless hordes outside.
Diplomats were ravenously devoured, providing almost sole sustenance for my fortress.
Diplomats never left unhappy.
That fortress eventually crumbled to its doom when a titan radiating kindness dodged down a vertical shaft into my fortress while attacking a caravan. The fall was not enough to kill the titan. The titan of kindness rampaged throughout the fortress, with all dwarves now trapped within the walls by the very drawbridge that kept them safe, unable to reach the lever as the titan tore dwarves apart, one after the other.
It kill my fortress with kindness!