(I fixed a minor BBCode error in your post, Frank, where there was an extra “bold” format. I only removed the extra formatting code at the beginning.)
Well, not only is the contrast not completely sharp when it comes to charity between the two Testaments (although quite a bit sharper when it comes to charity toward enemies), Jesus has some hair-raising words Himself on occasion in the Gospels! (Not even counting RevJohn.)
That includes, not incidentally, the Sermon on the Mount (including Matt 5 and chapter 7); Matt 11; and John 8. All three of those “contrasts” include harsh statements of punitive violence against Jesus’ enemies. (Romans 12’s “repay no one evil for evil” is predicated on the promise that God will do vengeance, too, citing a verse from the OT on that topic!)
But then, most of those two green paragraphs (starting from item #30) weren’t about contrasting the love and compassion of Jesus vs. the violence and vengeance of God in the OT. They were more about contrasting God being visible in the OT compared to not being visible in the NT.
Which is a trinitarian/anti-trinitarian dispute: trinitarian apologists like myself combine that detail with some other details about the visible YHWH, such as His occasional subordinate reference to a higher YHWH, with frequent citational identifications of Jesus as the YHWH of the OT, to argue that the NT texts present Jesus as being the person of God acting in pretty much all the famous and obscure OT stories. There are non-trinitarians who conclude that, too, in theologically different ways, of course. But the salient point for your comparison is that the NT authors treat Jesus as being the entity Who-or-who is so zorchy in the OT.
Having said that, when St. Paul cites Deuteronomy in Rom 12 about leaving room for God to take vengeance, the scripture he’s referencing is talking (as a prophetic promise) about God vindicating His own rebel people despite them refusing to repent of their sins: God may have to destroy them so utterly that they “are neither slave nor free” (a poetic way of saying they’ll be killed as far as anyone can be killed), but then the hardcore rebels will finally learn better, repent, and be restored to fellowship with God and man, with a happily ever after.
So the narrative and thematic contrast is not simply between seeking and not seeking revenge, nor even (a little less simply) between taking our own revenge and letting God take our revenge (which is the usual interpretation); but is about contrasting merely human revenge with divine revenge, because as sinners we can’t be trusted to really have reconciliation with our enemies entirely and fully in view, the way God does! So it’s better for other people, that we should not even try to seek revenge. (Which Paul doesn’t confuse with proper soldiering and police/magistrate work; he’s entirely fine with that in principle, as he immediately demonstrates in the next chapter, even when pagan authorities are the ones doing it!)
I did provide a detailed and nuanced answer about that at length in the comment immediately preceding yours. You might not agree with it (and/or understand it), and so not regard it as a sufficient answer, but it’s still an answer, and a coherent one to that question. It isn’t like my answer has nothing to do with that question.
You could adjust the question, though, to include the answer given and highlight what you’re still having problems about. But the narrative and thematic contexts are important, which is why I mentioned them in some detail.
Actually, by the cultural standards of the time, He did: the general call of having to hate one’s own family, the way it’s expressed, would have been shocking to any civilized ‘paterfamilias’ culture in that time and place, and would have looked especially outrageous to Jews since (on the face of it) it seems to run against one of the Ten Commandments!
True, He didn’t expect people to physically kill their parents (or even hate them, except as a rhetorical comparison to being loyal to Him–and the theological implications of calling for that level of loyalty!) But Jesus Himself pointed out that what one intends counts the same as what one actually does.
Relatedly, to one man in particular (apparently a scribe, per GosMatt), who asked permission to wait until his father had died, Jesus told him very bluntly to “let the dead bury the dead” and follow Him instead. In the cultural standard of the time, refusing to care for the body of a parent after death would be tantamount to murdering him.
So the underlying refining test of such acts is still there, even if the acts themselves are physically non-violent.
(Besides which, as noted, the NT authors have no problem treating Jesus as being the same YHWH Who ordered the OT heinous acts, including the sacrifice of Isaac.)
In the OT, YHWH also nukes Israel via the horrors of invading pagan occupiers (although He also makes a point of treating the invading pagan occupiers as villains who will be punished for, in effect, volunteering to nuke Israel), when (1) they refuse to worship Him and (2) they refuse to be self-sacrificially charitable.
Rome, as everyone agreed, was the latest in the long-running conga-line of pagan oppressors, permitted and even directly sent by God to punish Israel for infidelity. The whole operational rationale of the Pharisee party was to lead Israel to be faithful to God, by means of faithful Torah observance, with the expectation that if Israel was ever even once, even for one day, faithful to God by means of Torah observance, they would thereby merit the salvation of God and God would send the Messiah to destroy the Romans.
Instead the Presence of God shows up, exhorting them (1) to recognize and worship Him, and (2) to be self-sacrificially charitable (even toward their enemies, the hated Roman occupiers). He doesn’t threaten Rome, He threatens Israel and Jerusalem in particular with destruction if they don’t accept Him and commit to fulfilling fair-togetherness with everyone, being a light on the hill for the nations. Forty years after they reject Him and hand Him over to the pagan oppressors to be crucified, they finally try to force their way out of God’s ongoing punishment (Roman occupation), and Rome zorches them, increasing their punishment from God. (And then about 50 years later they rebel again, with a false Messiah leading the rebellion, and they get zorched again, for the final time.)
Like it or not, that’s fairly typical behavior for God in the OT. There are some notable extensions and differences – the Presence comes humbly as a man born among men instead of as the Glory of the Shekinah (or even instead of a mere human/angel manifestation), and fair-togetherness toward enemies is emphasized significantly more. But in other regards it’s business as usual, not “the complete opposite of anything in the Old Testament”.
This, by the way, is why Marcion wanted only to include the Gospel According to Luke and the Pauline epistles–but heavily altered and edited ones! (He isn’t credited with “the formation of the New Testament books”, but he is the first person we explicitly hear about in the surviving records who tried to authorize a proper collection of what does and doesn’t count as canon.)
Isn’t that a different topic? 
The first Christians sure didn’t regard it as an oxymoron: they (including Jesus) taught that they were the fulfillment of Israel and Judaism, and Jewish religious concepts are heavily promoted and accepted throughout the NT (albeit sometimes in ways that aren’t very obvious to us Gentile readers many centuries later). St. Paul warns his Gentile audience in Rome (Rom 11) that they had better not despise the Jews who hadn’t accepted Christ yet, because they are the ones into whose promises we’re grafted. They might be currently grafted out of the vine, but they’re natural to the vine and so will fit even better than Gentiles do when they’re grafted back in; whereas, if God does not spare even branches natural to the vine but grafts them out, neither will He spare Gentile Christians grafted into the vine if they misbehave!
Sure there are critiques of proper Jewish behavior, especially in light of Jesus as the Messiah, but those are critiques from within Judaism as to how best to be properly Jewish.
While I agree with Cindy about Christian Muslims and Christian Buddhists, in that there can be a true Christianization and baptism (so to speak) of their culture, no one can be a Christian Muslim or a Christian Buddhist in the sense that Christianity is fundamentally Jewish. Even the Christological disputes were originally (and still ought to be) how best to account for and reconcile Old Testament scriptural testimony about God and God’s Messiah along with New Testament scriptural testimony about God and Jesus the Messiah.
Christians haven’t been very good about respecting that connection throughout our history, but major steps have been taken among scholars (mostly Protestant and Roman Catholic) in the past forty years or so to reforge those connections, and to teach respect for them among the laity. I can recommend some Protestant and Catholic books on the topic, if you’re interested. (Both of the most recent Popes, John Paul 2 and Benedict, helped contribute to the Roman Catholic side of this reconnection.)
), has mercy and reconciliation of His enemies in view. It takes some digging and narrative/thematic analysis to find it and put it together, but it’s there–maybe not in as much proportion as in the NT, but my impression is that there’s more of it in bulk because the OT is that much larger! 
)
) :