The genocidal God of the Abrahamic religions

As an atheist brought up in a Christian household and educated in a Christian school, I - like many of my contemporaries- can recite the genocidal atrocities carried out by the God of the Old Testament with some ease, and I never cease to be amazed that anyone can worship a deity capable of the heinous and disgusting crimes against his own creation related in lip-smacking, blood-curdling detail in the "Good Book". 
Muslims of course smugly distance themselves from the bloody destruction of the manically jealous God of Exodus (killing the first-born of Egypt, wiping out of the Canaanites, Hivites, Perizzites, Jebusites et al) and Numbers (slaying of the Medianites with the specific exhortation to "kill every woman who has ever slept with a man" and the Medianite boys!) and Deuteronomy wherein infamously God told the Israelites to put to the sword everyone in any town where they worship a god other than him: "Destroy it completely, both people and its livestock"...) I could go on and on, but you get the picture.

The awful stuff related in the Bible can't be laid at the door of Allah, Muslims say, because the scriptures of the Christians and Jews have been tampered with. My convert friend explained it to me some years back like this:

Because Muslims believe that religious scriptures other than the Qur’an have been changed over time, we do not take these other scriptures as sources of evidence. Therefore, where they discuss matters that are not contradictory to Islam, we neither believe nor disbelieve in them. We merely note their discussions as points of interest.
 So if my friend is typical (and I have no reason to think he isn't) then Muslims - should they even be aware of the disgusting murderous sprees enjoyed by the Judeo-Christian god as recited in the Bible - will note the blood-letting, the infanticide, the rape and the slaughter "with interest" and move on, presumably telling themselves that these particular passages are the ones that have been changed over time. Surely the god of Islam has been misrepresented, they doubtless tell themselves. Surely the "Most Merciful" wouldn't go around telling His people to bash babies' brains out ("The people of Samaria must bear their guilt, because they have rebelled against their God. They will fall by the sword;">their little ones will be dashed to the ground, their pregnant women ripped open." Hosea 13:160.

There is no doubt that many Muslims are keen to throw doubt upon Allah killing the whole of humanity in what must be the most audacious and awe-inspiring crime against humanity in history when He drowned everyone except Noah and his immediate family. Modern Muslims now claim it was purely a "local flood" - more credible perhaps but morally only slightly less despicable...

But what of the wiping out of entire nations because of some apparent slight perceived by the lunatic jealous deity? I bet that's one Muslim readers are keen lay at the hands of the evil redactors: "Yeah, well - I note the homicidal rage and genocidal tendencies with interest, but I can't really say I believe it ..."

But Allah is actually quite keen to have us remember that he was partial to a bit of genocide in the Qur'an. For what are we to make of 21:6?

 Not one of the communities that We destroyed in bygone times would ever believe [their prophets]: will these, then, [be more willing to] believe?

Or 21:11?

 How many were the nations We utterly destroyed because of their iniquities, setting up in their places other peoples?
If we take these verses at face value then it seems pretty clear that the god of the Qur'an is the same murderous psychopathic genocidal one of the Old Testament. But whereas liberal Christians can admit the OT is a collection of myths, Muslims MUST take every word of the Qur'an as...well gospel.