Umm...theological giant? Hardly. Some thoughts, though...
There was the separation of the two testaments in the VERY early Church (Origen, I think).
The main reason that we use the OT, obviously, is because it points to Christ. That was the catechism of the first believers; they used it (as Christ had) to present a picture of who God was, along with the plan of salvation as traced through history.
As for the gory parts...well...I'm surprised your friend didn't mention Judges or the "waters of Babylon" Psalm where we celebrate the idea of bashing the heads of wicked men's children on the rocks, but...the idea is troublesome. It helps to look at the OT as being incomplete, like it's waiting for something to happen (which is was). Christ said He came to fulfill the Law/Prophets (Matt. 5:17), to give it its "cadence" or "resolution," so to speak.
Paul said the Law (and prophets, by implication) was a tutor (Gal 3:24-25) and needed replacing by faith in Christ (v. 26).
Since we see these (and many other) references to the Law undergoing a shift, it stands to reason to ask: what kind of a shift, and why?
The shift seems to be this:
OT: The Isrealites were called to be a witness of God's power within a world that valued land acquisition and cattle quantity, etc. as signs of your God's power and favor; the campaign through Canaan thus seems appropriate for the time. Also, there's the fact that this land Israel was to inheirit (sp?) was pretty much overrun by tribes who worshipped everything under, and including, the sun, and committed all kinds of lovely atrocities in the name of their god, the tree stump -- the most obvious parallel to the NT with what Israel did to them (i.e., kill to the every last man, woman and child in most cases) would be what St. Paul calls, "giving them over" to their desires (Rom 1:24), or basically, giving them the end result of what they were pursuing anyway within idol worship/child sacrifice/orgiastic sex worship, etc.
NT: The parallel becomes internal. Finally, the weakness of the Law to transform the inner man is replaced with a law of grace that works man from the inside out. Now the landscape is not a physical area of land, but an inner area of the soul. Now there are different enemies -- demons, our passions, etc. -- who want to do things just as bad as (if not worse than!) those the Israelites drove out. We are to be just as merciless towards them as God commanded the armies of old to be towards His enemies. Like everything else, the violence of the OT has not only been used to glorify God during the time it ocurred, but also during a future time of which those who chronicled Israel's journey were not aware (1 Pet. 1:11-12).
Hope my rambling answered some questions?