Why is the sin nature inherited?

[System of Solace]

Once Adam and Eve sinned, they became imperfect as you said, so imperfect offspring were inevitable. Sin effected every corner of the creation, it put everything in a state of corruption. Adam and Eve were told by God to populate the world before the fall, hope this answers your questions.

And God would have known they would have sinned. He would have known Satan would try to tempt Eve. So, admit one of these three:
1. God is not omnipotent.
2. God is a major asshole.
Or, if you want to be logical:
3. Everything in the bible is complete bullshit. Especially the old testament.
I dare you to say it was God's plan. Do it. DO IT.