How can we square the old testament God with Jesus?

I have been reading the Old Testament and was surprised to learn that God ordered Moses to have the Israelites slaughter every man woman and child when they were driving the various peoples out of the promised land. I understand that God had no mercy towards Idolaters and did not want the Israelites tempted into idolatry but slaughtering everyone even the children seems so harsh and at complete odds with what Jesus taught.
In other words how can Jesus who is the same God as the God of the old testament be so different from Him?

I am a 76 year old cradle Catholic and am sure I am not the first person to come up with this question. Please recommend something that I could read that addresses this issue.

Thanks,
Dave Bayer

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