Personally, I do not understand how anyone can criticize Hitler while honoring the genocide recorded in the Old Testament.
Joshua's brutal slaughtering of families; including babies, children, mothers, and the elderly, while raping the young "virgin" girls seems pretty much in line with other powerful modern day leaders of whom you speak.
Seems to me it could be argued that those who believe in the Bible, do, at least to some extent support the idea that those who believe differently, or those not of the chosen lineage should be wiped off from the face of the earth. Certainly there is the belief that there exists a chosen race. (The foundational element of Abrahamic religions is that one must be a part of his tribe to be counted as a chosen child of God, or to receive eternal reward).
I suppose one difference would be that Moses and Joshua claim God told them to do the killing?
1. The clear majority of ancient Israel's wars were defensive in nature, as they were continually under attack for much of their history. Those that were aggressive in nature were understood to have been authorized by the Lord as a means of destroying peoples who were and an apogee of wickedness and who were "ripe" in iniquity and (and who perhaps could be foreseen as a potential future threat, given those cultural dynamics) God removes a people from the scene under those circumstances.
2. Such "ripeness" in iniquity and its consequences (civilizational collapse usually followed by devastating wars) are common to the Book of Mormon, much of which takes place in an Old Testament context. That the Lord uses other peoples as agents of his judgments, on some occasions (or the forces of nature on others), is as common to the Old Testament as to the Book of Mormon.
3. The Israelites did not engage in "genocide" in the modern sense. They obliterated certain discreet cities and city states, but not entire peoples.
4. They did not engage in the few wars of clear aggression mentioned in the Old Testament because others believed differently from them. These were not "holy wars" in the Islamic sense. They were not wars of forced conversion or even of conquest and subjugation, in the normal sense. They were understood to be instruments in the hands of the Lord of executing judgment upon a people who had ripened in wickedness and were at their moral and spiritual end, as a culture.
The Israelites were not warlike per se, and did not glory and wallow in cults of warfare or violence as did many of the peoples around them. They had a "just war theory" that limited them to the extermination, when authorized by God, of peoples that had become "ripe", and to defense of their own kingdom.
All the modern examples I've used involve naked aggression for the sake of conquest qua conquest and hatred of entire classes of human beings due to either inherent or acquired (class) characteristics, none of which fits the Old Testament context of Israel's military and social history.
Regardless, much of this, as was the Law of Moses itself and its harsh characteristics, was fulfilled and done away in Christ. War still exists, and nothing precludes Christians from going to war for just reasons, even, as the Book of Mormon clearly demonstrates, to the point of large scale extermination of relentless enemies who will not live peaceably and refuse all attempts at peacemaking and coexistence.
I find your insinuation that there is, within Christianity, some inherent tendency toward violence, war or genocide to be just a bit to the left of preposterous. The only time this has ever manifested itself on a mass scale are those times when church has been identified with state (at which times, the state always dominates, at the end of the day).
God utterly destroyed Sodom and Gomorrah through some natural disaster as well, including small children and new born infants. Can God do this? Of course he can, as he is the ultimate giver and sustainer of life, and the mere preservation of physical existence is not his highest priority under certain conditions, among which are "ripeness". Acceptance of God's action here does not have any relation, in a gospel sense, to an authorization to take any action of our own under similar circumstances, quite obviously, as what God does, unless he directly delegates a specific kind of authority to humans on earth, is no license to imitation among mortals.
It is the vary fact that the modern examples I've used, both Nazi and international socialist, involve, indeed, human beings who raised themselves to the position of gods and anointed themselves as arbiters of the ultimate fate of other human beings.
There is no question of deriving authority or sanction in the secular sense, from the God of the Judeo-Christian tradition, nor is there a "just war theory" in the Nazi or communist world.
The means justify the ends.
You are making an apples and oranges comparison, if I haven't made that clear already.