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Prophet Samuel explicitly states that the Amalekites should be exterminated and reprimands King Saul for not doing so completely. Under the doctrine of "thou shalt have no other god before me", the first and most important commandment, it can be extrapolated that none shall be allowed to have a god other than YHWH, and Samuel's command--with implicit religious backing

--shows that the extermination of all those who do not follow the true god and true path is religiously moral. By the most conservative estimate, that would require the genocide of all non-Abrahamic religions, but could easily be recontextualized to be understood as the requirement for the extermination of all non-adherents to a specific creed of Christianity.

Jesus's commandment to "love thy neighbor" can in this mold be understood as to include true Christians only, as the Amalekites were neighbors of the Israelites and Saul had divine permission and commandment to unprovoked aggressive extermination.

Furthermore, "thou shalt not kill" is in direct contravention to the Samuelite interpretation to the first commandment, and the prioritization of commandments in the Mosesite legend makes it pretty clear that YHWH considers loyal worship more important than respecting human life.

Thus, Christian argument for genocide.

The problem with your argument is, the destruction of the Amalekites was expressly ordered by God as punishment for the crimes of that tribe. God, of course, is master of life and death and can choose whatever implements He wishes to inflict punishment. God does not owe us anything, and it is out of His own goodness that He extends mercy to us.

Of course, it suffices to say that we Christians have never received such a direct command from God, and therefore cannot presume to carry out such thing in His name. In that case, it would be a horrible crime. As for "loving your neighbor as yourself," the notion that this should only apply between Christians is contradicted by Jesus in Matthew Chapter 5:

Likewise, Acts 13 ("For so the Lord has commanded us, ‘I have made you a light to the Gentiles, that you may be an instrument of salvation to the ends of the earth.") and the entirety of Romans 11 ...