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.

Do you have any recommended reading on the history of Christianity? I find it challenging to find unbiased / purely historical explanations of how we got to this point.