An accurate shot with a gun is probably the most humane way to kill them, if they are going to be killed at all. I once asked a pest control professional what happens if you trap them and then hand them over to the pest control He said that in that case a stick is poked through the cage and they are pulverised to death - and that's after they have had the suffering and fear of actually being trapped for up to several hours. Others I would guess might drown them - which is slower than shooting or gassing which burns the lungs depending on what's used. At least with the gunshot the squirrel thinks it's free right up to the second it dies.
He never gives up being stupid.

