So, God was confronted with a dilemma. On one hand, He had already decreed that only those who are perfectly righteous are deserving of eternal life. Thus He had, in effect, demanded that a very high price be paid for billions of unrighteous human lives. That price was billions of eternal human deaths. On the other hand, God wanted to give every human being the gift of eternal life, even though none of us deserved it, and even though His own high standards prohibited him from giving us that gift.
I don't know why God didn't just lower his standards for who deserves to live? Especially considering the fact that he created us in such a way that it was completely impossible to meet his standards in the first place. The entire thing seems primitive and contrived, IMHO. Sitting here trying to reason it out is about like trying to work out the exact location of Santa's house based on patterns of present distribution.