People reading the Old Testament for the first time are often shocked by how merciless God is. This is more because they reject the God Jesus professed than anything else. In most cases this is because they were denied the gospel in context, and given a wishy washy fairy tale about a God so weak and ineffectual that it is a surprising miracle he created the world or anything at all, the fool he must be.
They never believed in Christianity or its gospel, and thus instead of gratitude, sit in judgement on God. God is deeply merciful, because God is just. But those who preach the gospel of mercy without the gospel of justice will likely spend eternity in the torture of hell, all the more if they are a priest or bishop. To preach one without the other, a New Testament God who would thrust the Old Testament God into hellfire for his crimes, is to preach not just a false God, an idol, but one so pathetic that Jesus' death on the cross is more like a creepy stalker thing done by a spurned lover, than an act of mercy from God.
The mass killing, jealously guarding God of Moses, who swallowed evil men, their wives and little children alive into the Earth, is the God Jesus died on the Cross and rose again for.