This question has been the subject of many a book, too many to even attempt to make a list of them. But I’ll try to give you a shorthand answer. The Biblical God seems to have a penchant for war regardless of if Israel is the beneficiary or the victim of it. It’s all over the Hebrew Bible (HB) and the Christian Bible (CB). Any honest reader will certify this, but some readers, imbued by anti-Jewish sentiments, would say that that maybe true in the HB but not in the CB. Jesus was different, they would say, he never condoned violence or war. He gave us a different interpretation of the Biblical God, one in which love of God and love of neighbor were the prominent features. Be that as it may, no one can deny that there are times when Jesus seems to have encouraged violence, for example when at Gethsemane he asked the disciples to sell their cloaks and buy swords (Luke 22:35-38), or when he stormed into the temple and wreaked havoc among those who were providing a service to the worshippers, never mind that they took advantage of their position (John 2:13-17). But still what Jesus did was violent, it was more like a riot than to a liturgical interruption. Add to that the times he talked about the eschatological end-times warning people to behave in such a way as to avoid the fire of hell, and the result is a not-so-peaceful Jesus. One particular case that comes to mind is Matthew 25:31-46. And there are others.

On the other hand, it is true that Jesus brought a refreshing look at the issue of peace, and peacemaking, when in the Sermon on the Mountain he (or was it the evangelist?) placed it in the context of other virtues, all of them countercultural (Matthew 5:1-12; Luke 6:20-26). And, of course, there is also the Gospel of John with its reverberating theme of peace rolling over its pages.

So, here is the shorthand answer to your question: when God initiates the violence it is somehow justified, after all, who are we to question God? But when humans start it, then it is condemned. Who are we to usurp God’s prerogative? A typical passage that comes to mind is Romans 12:19: “Vengeance is mine, I will repay, says the Lord.” Contradiction more than agreement seems to be the answer to this dilemma and I hope you can learn to live with it.