You shouldn't try to make sense of it, OP.
If it made sense, or was well-evidenced, believers wouldn't need to have faith.
To those of us without faith, it's just nonsensical (and a bit unpleasant).
God is, purportedly, all-knowing, all-powerful, benevolent (all-good) and, of course, created everything. This means that, when he made the world, he not only knew that Adam and/or Eve would commit the original sin - it was God's choice that they would do so (if he hadn't wanted them to, then he could have just make them or the world a little differently).
God knew about every subsequent sin that everyone would commit and, given that he chose to create the world and it's people that way, he essentially chose for every sin that had ever occurred, to occur. Free will isn't really reconcilable with Christianity, to my mind (though some have tried to offer explanations).
He also always knew that humanity would "need to be saved" from its (his) sins. Despite being all-powerful, meaning he could have waved the sins away, the specific way he decided to do this saving was by having his son (who is also him) killed.
It's very hard to understand if you're not coming from a starting point of believing it regardless.