When the water temps drop the metabolic rate lowers. At a certain point reduced blood flow to the brain induces coma, then the organs shut down and the the heart stops. There is no over stimulation to the nerves.(what we call pain)
As death go that's about as low drama as it gets.
Nothing other than instant traumatic destruction of the brain is more humane.
I put the fish in a small bag of water in the freezer before I leave the house for the day. When I return the frozen bag & the deceased go out to the trash and life goes on.
When I was breeding cichlids I had an Oscar that was my Quality Control Mgr. he handled the culls.