Don't you believe that someone has a choice to be saved or not? If so, how is that god alone saving someone?
The Bible tells us that no one can come to Jesus if that person is not called by the Father. It further says that the way to the Father is only through Jesus. It also says that Jesus is the "author and finisher of our faith". Our only responsibility is to receive the free gift of faith. But, if we turn away from all that God has done to effect our salvation (like the Rich Young Ruler--and you, if you are telling the truth) then, there is no more remedy for sin. I believe that such a one can repent and beg God's pardon, but it is a dicey position that one puts themselves in when they turn their back on Him. After the Rich Young Ruler walked away, Jesus mused that it was quite difficult for a wealthy person to enter the Kingdom of God.
God's justice demands payment for sin. Either Jesus pays for our sin or we will after death. Even in this life, we often pay some price for our sin.
If you are correct, then all children including babies are responsible for their sin and deserve death since it says to kill everyone.
News Flash: NO ONE gets out of here alive. Some die sooner, some later but all die because sin and death was let loose on the planet with Adam's deliberate disobedience. Your system only works if this life is all there is. Jesus said that the "angels" (a way of speaking of someone's soul) of small children "are ever before the throne of God". They are in heaven. And THAT is much better than being raised by horrible heathens.
But, Deuteronomy 20 doesn't say the "Hittites and the Amorites, the Canaanites and the Perizzites, the Hivites and the Jebusites" were killed for their sin but so that the Israelites would not learn their ways and sin against god.
So? That is true. Aren't you glad that your parents weren't into child sacrifice? How about the Greco-Roman practice of just placing a superfluous new baby out for the wild animals to drag off, just so they wouldn't have to be bothered with him/her. Thank Jews and Christians for always fighting against this scourge on the planet. Just as they now fight against abortion on demand.
God ordered the killing of all these people so that his chosen people would not sin against him in this way. Even though they were going to sin anyway. Why not just tell the Israelite's not to do these things? Well he did in Duet 18 and I guess he did not trust them to not do them.
He was right not to trust them, wasn't He?
Would this be a reasonable way to stop people from sinning today?
The Bible suggests that, in the Millennial Kingdom, Christ will rule "with an iron rod" that "only sinners" will die at 100. Perhaps instant death will happen to those who violate God's Law in the Millennial Kingdom, which is obviously a mortal kingdom since physical death will be present and it speaks of the "sons" of the "Prince" in the latter chapters of the Book of Ezekiel. If they were resurrected beings, they would not experience death and Jesus says that resurrected beings do not marry (and by extrapolation, they do not have children).