You ask good questions, but if you’re really interested you can look into Christian apologetics re: free will. There are some interesting answers awaiting you. But the gist of it is that God didn’t create flawed beings, he created beings with free will that chose to be flawed.

And Christianity has never said free will is a flawed design, because humans having free will is one of the most important aspects of the religion and is very fundamental to what it means to be a human (a concept that is true both in and outside of Christianity, unless you believe in destiny or something). It is not a flaw to have free will, otherwise God himself would be flawed. In a regular context, it’s kind of like you’re not flawed for existing, but you’re flawed if you do negative things with your existence. I would personally have to be convinced that having free will is a flaw/a negative thing

To quickly answer your first couple questions: death is the punishment for sinning and Jesus is supposed to be perfect and sinless and thus should not die. but instead he died in place of other sinners, kind of like taking the blame for them. And yes, torturing and killing the son of God was indeed a sin, the people who did it were sinful. I don’t think anyone has said otherwise. The ones who killed Jesus were not his followers or supporters