That really is a red herring. It is abundantly clear in the Scriptures. What Calvinists and non-Calvinists are arguing about are much more subtle points of theology. We pretty much all agree that salvation and faith come by the agency of God alone and that we can do nothing to earn either one.
Don't you believe that someone has a choice to be saved or not? If so, how is that god alone saving someone?
The people whom He ordered killed were wiped out in order to protect the Israelites from the absolutely horrible people around them. Read Genesis 18 to understand some of how corrupt and deviant the Canaanites were. The Ras Shamra documents (written by their own heathenish hands) have revealed further about these demented, depraved and demon-infested people.
If you are correct, then all children including babies are responsible for their sin and deserve death since it says to kill everyone.
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.
"but the Hittites and the Amorites, the Canaanites and the Perizzites, the Hivites and the Jebusites, as the Lord your God has commanded, that they may not teach you to do according to all their abominable practices that they have done for their gods, and so you sin against the Lord your God." Deut 20:17-18 ESV
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. Would this be a reasonable way to stop people from sinning today?