We saw him get killed at the beginning. When he thought he had killed that other guy and turned his back on him to gloat with his friends.
I don’t think the writers are trying to answer the question ‘nature vs. nurture’ but just look at it from different ancles. And the adoption aspect ‘growing up out of your nature’ is one ancle. Henry for sure have some of his biological parents in him. How about Emma though, does she have any Charming attributes ?
You’re right Snow allways believes in the good in people. But maybe that makes her blind to some things. I don’t think she can comprehend the thought that David could have been as bad as his twin brother.
I think that it is a romantic idea that people are born with a surtain kind of ‘soul’ or heart or whatever. And I doo think that Snow believes in that.