This is a common accusation that arises even from a cursory read through the Old Testament. Over and over we read that God demanded the Israelite armies to commit genocide. Of the Amalekites we read that God commanded they should "utterly destroy all that they have; do not spare them, but kill both man and woman, infant and suckling, ox and sheep, camel and ass." (1 Samuel 15.2). Further, Exodus 15.3 says outright that "God is a man of war." With these indictments and the weight of Old Testament scripture, how can a Christian even suggest that God is a god of love (1 John 4.16)?
There are a number of responses the Church has made over the years to deal with this very topic. Perhaps the most common response suggests that the advent of Jesus Christ made a significant impact upon not only the people of earth, but with God as well. This response suggests that Jesus ushered in a kinder, gentler God who was more interested in redemption and relationships with God's creation than in retribution. Romans 15.33 (and many other passages) says that God is a god of peace, while Romans 5.1 says we have "peace with God through Jesus Christ."
The problem with this response is Hebrews 13.8 says Jesus is the same yesterday and today. Add to this the assertion that Jesus existed from before the beginning of the world (John 1.2-3) and we're left with no good reason why God suddenly became "gentler" following Jesus' birth. He was, after all, with God from the beginning. So why didn't he "temper" God's wrath from the beginning? Another problem with this response is the image of God and Jesus Christ during the "end times" as portrayed in the book of the Revelation. There we see the godhead visiting their wrath upon the earth through plagues, wars, and both natural and unnatural disasters. In the end, the kinder, gentler God seems to vanish.
A second response traditionally offered is that God is God ("I am who I am" Exodus 3.14) and we have no place questioning God. Personally, I am not one to dispute God's sovereignty. If God wants something done I will be one of the first to volunteer. On the other hand, since when is genocide an act of love? In the conquest of the Promised Land by Israel, one could perhaps argue that love was being shown to the Hebrews as opposed to the Canaanites. Thus, God may be multifaceted, much like humans are. On the other hand, one old Jewish story about God tells of the exodus escape across the Red Sea and the pursuing Egyptian army. When the parted sea collapsed on the soldiers, killing them all, the angels in heaven were rejoicing. But God says, "How can you sing when my children are drowning?"
One of the mantras of sociologists and anthropologists is that gods are created in our own images. There may be a modicum of truth to this indictment. In my own lifetime I have heard sermons about how God had commissioned America to invade Vietnam and bring about the downfall of evil. And, according to many Church documents from the Civil War era, God endorsed slavery. Many Native American tribes were disposed of in the name of missionary zeal. And how many Muslims were slaughtered in the God-commissioned Holy Crusades? Certainly, since the days of Christ, the word of God (and the scriptures) have been twisted to justify war after war and atrocity after atrocity. But did God command these atrocities? I don't think so.
So, is war God-ordained or human-justified? Many suspect it may be the latter. Therefore, another response to the war-god indictment is that the pronouncements by the ancient prophets (old style preachers) were less God-ordained and more politically motivated. Thus, God may not have advocated war, let alone genocide; instead, God was "credited" with the desires and the acts of a nation.
So, which is it? Is God a god of war or a god of peace? Jesus claimed to be the ultimate revelation of God - one who visited earth because humanity kept getting it all wrong. And Jesus claimed that God was a god of peace. Period. But is this just one side of a two-sided God? Perhaps. Ultimately, as usual, it comes down to a matter of faith. Personally, I'll leave the matter of war and other evil stuff to the powers of darkness and embrace the God of peace.