The sin of Adam broke the relationship between God and man .
Adam was the representitive of all of us and he failed....We were all present with him in the garden when he sinned through his seed....so we all carry the sin of Adam (original sin).
We are sinful because we will to be sinful. Not beacuse God made us that way.
The question to be asked and the one you see discussed here often is how much damage was done by the fall.....was it a scratched knee or was it fatal?
I know that as a LDS this is a strange concept to you because you see the fall as "good "and necessary for progression.....but for a moment think of willful disobedience /rebellion as sin..How does God react to sin?