To be humane you could use a .300 Winchester mag. They would just explode so death would be instantaneous. No soup or stew then.
If you are a good shot and have a larger caliber rifle, wait thill they come out to sun themselves and just pick um off.
I wouldn't want folks to think I left the turtle to suffer. I saw a crow get hit yesterday and not kill him but did hurt him. What should I have done?

I wouldn't want folks to think I left the turtle to suffer. I saw a crow get hit yesterday and not kill him but did hurt him. What should I have done?