You asked a question I answered it.
Is a 50 gallon big enough for a ten inch cooter who has probably been alive for at least 5 years in the wild to attain that size....I answered. No. It is not.
You are the odd person here. Not us. We do not go out and catch adult turtles from wild populations. I am sorry you are upset, but its not your feelings we care about. Its the turtle. And frankly the best advice anyone can give you on this forum in regards to morals, whats right and whats best for the animal, is to let it go.