Note that Christianity is a religion that was grafted onto a previous one that is entirely different by intended recipients and worldview. Christianity is a universalistic religion centered on mercy and forgiveness, ancient Judaism is a transactional pact between God and one People: God gives the land and protection, the People worships God and follows the rules. I was in a church a few days ago and it was almost funny how the priest read from the Old Testament and turned a quite literal passage about the People of Israel, protection from regional enemies and the promise of a kingdom into a metaphor about all humanity and the fatherly care of God towards all. Unfortunately Christianity decided to incorporate the Old Testament and read it as a metaphor and a prophecy, and this allows some Christians to revert to a language of violent conquer and triumph against the enemies whenever it suits them.

The strain(s) of Christianity we have today are in large part the result of chance and power plays, it's a fascinating history.

More to your point: the Old and New Testaments are so different that a significant proportion of Christian apologetics consists in finding ever more convoluted mental hoops to explain this away.

A few centuries after JC, one of the dominant interpretations, Gnosticism, held that the two Testaments were about two different gods. The old god was angry, cruel, capricious, sadistic, while the new god, as described by Jesus, was the exact opposite. One ordered populations to be killed for nothing, the other turned the other cheek. The Gnostics thought that the first god was the demiurge, the god of matter (like Morgoth), while the second god was the supreme god (like Iluvatar).

A much simpler solution to this dilemma. Unfortunately, the non-Gnostics were better at organised religion/politics, and the rest is history.