My headcanon is that the Old Testament was a god specifically of the Jews and some upstart god took over (possibly after murdering him) in the New Testament days and then proceeded to spread his influence to non-jewish people while aggressively eliminating any opposition. Wherease in the old days people believed in various gods, this one started of campaign of montheism, depriving the rest of them of faith and eliminating them one by one. Nowadays he's kicking and screaming because the rise of atheism in many parts of the world which used to be his strongold is depriving him of energy and thus he finally faces annihilation.