Hittites, Amorites, Canaanites, Perizzites, Hivites and Jebusites might have something to say about that.
I think you're comparing apples to oranges, Jamil. Different times, different covenants, different judgements. No where did Christ give leeway for us to do anything but live our neighbors as much as we love ourselves. In a much earlier time, destruction of the inhabitants of the promised land was a result of God's judgement of those peoples' long standing sins despite God's warnings.