A sacrifice was necessary, because God is holy and just. If he just overlooked sin, he wouldn't be holy. And because God would not punish another creature for sin committed by humans, it had to be a human who who would mediate and bear the sin (I'm using this language because I think it's important to mark the difference between the kind of sacrifice Christ made, and the vile "human sacrifice" of pagan practices).
And yes, God knew about all of it in advance, and it all proceeded according to his plan - the test of Adam and Eve in the garden, their massive failure, and Christ's redemption.
I have no personal problem with you if you choose not to believe in Christianity, that's up to you. But if you don't believe something, just say so. You don't have to misrepresent what Christianity is. I hope you don't mind my comments.