Question:
Please see Divrei Hayamim 1 20:3 Was this genocide? If so, was King David justified?
Answer:
See Ralbag who explains that the reason that David did this was to send a message to all of the other nations not to start up with the Jews. As a side point, the people of Amon worshipped Molech, which was worshipped by walking their children between two fires, and burning them. Therefore they were all guilty of capital punishment for worshipping Molech.
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