Genesis 6:4 speaks of children who were born of an unholy union between fallen angels and human women. On the whole Genesis 6 implies these children were evil and perpetuated evil and the whole world became corrupt to the point of God having to start humankind all over again with clean DN
I think you’re making a number of assumptions for which there is no underlying support. For instance, do you know that these hybrid “children”, called Nephilim, even had spirits? Or that they are suffering eternally in hell? What if they were simply destroyed in the flood? Also, there’s no provision in the Bible for the redemption of angels. Why should there be a provision for the redemption of their illicit offspring? If God only made a provision for the redemption of mankind, doesn’t that tell us He didn’t consider the Nephilim to be human?
But most importantly you’re suggesting that perhaps God was not just in His treatment of them. And yet in Deut 32:4 Moses said of God, “He is the Rock, his works are perfect, and all His ways are just. A faithful God who does no wrong, upright and just is He.”
I think this is a case where we have to trust God to have acted justly in a matter that we know almost nothing about.