So, you're a Victorian who admires ancient Greece and Rome, but you want to denigrate modern Greeks and Italians. There are two main avenues open to you:
Both approaches are time-honored, and both have many champions.
Prior to the Victorian period, #2 was probably more popular, and you'll have encountered it in writers of almost any genre. In the 18th century, when there was an upsurge of skepticism generally, this probably took an anti-Chrisitan line; in the 17th century it was more likely anti-Catholic; but both arguments were common and were mixed together.
The line as advanced, probably most famously by Gibbon, goes like this: the ancient Greeks and Romans were free, virtuous, and manly; but then they converted to Christianity. Christianity, of course, inculcates passivity and resignation and is opposed to strenuous effort (because you should just pray to God and accept whatever is is demonstrated will). This zaps a society of its vigor and results in the sloth, laziness, and crime (they thought) that you see in Italy today.
But England is also Christian; how could they denigrate Italians but not the English? Well, because of course Italy is in a sense more Christian: more priest-ridden, with more monasteries and convents and abbeys, not to mention inquisitions. In England, of course, you got rid of all that nonsense, which is why England is better than Italy. (Conyers Middleton is an excellent source of this attitude.)
That leads into the anti-Catholicism: why is Italy so backwards (they thought), and England and Germany so progressive? Because the latter are Protestant, of course! It isn't Christianity in general, it's just Catholicism, that retards civilization.
But you're a Victorian. You can still take the anti-Catholic line; and a lot of Victorians do take the anti-Catholic line. (Macaulay, who was rather tolerant of Catholicism by the standards of his day, still writes of Italy as a bastion of superstition, in thrall to the Papacy.) But as a pious Victorian, who have left behind you the skepticism of the 18th century, you probably won't take the outright anti-Christian line.
But you have another argument open to you. With the growth of science and historical research in the 19th century, and especially the explosion of philology, which traced the origins and changes of languages down the years, and with the increasing interest in tracing the 'migrations' of populations in antiquity by where their languages spread -- now you're more likely to argue that the Italians (or Greeks) aren't actually the descendents of the ancient Romans (or Greeks). No, there have been all manner of barbarian invasions and settlements, and hardly any of the 'genuine' blood of the ancients is left; it has been diluted by the 'inferior' blood of all these late-comers.
It's hard for us to imagine today how controversial this was, and was until very recently. Some of the most intense scholarly fights were around the ancestry of modern nations; even moderately educated people felt emotionally invested in these debates. Were the English the 'pure' descendents of the Anglo-Saxons, or mingled with Celts and Britano-Romans and others? Were the Greeks truly the descendents of the Athenians and Spartans of old, or had they been exterminated in the Slavic invasions of the middle ages, and truly there were no 'real' Greeks left? And the Italians -- how much of the good old Roman stock was left, amid the dregs of the Goths and Lombards and Sarmatians, and in the south even *gasp* Arabs and Africans?
So, if you want to denigrate the Italians while admiring the Romans, that's not a problem! The Romans were a good old stock, but their blood (you can say) has since been diluted beyond recognition, corrupted by the admixture of all these foreign nations. After all, didn't the Romans themselves complain of this in antiquity, that all the dregs of the world (Syrians and Greeks and Egyptians, shudder) ended up in the common sewer of Rome? And if the Romans themselves believed that little Roman blood was left in Rome in antiquity, how much would be left after eighteen more centuries of corruption?
This is all, of course, super racist. But it's the start of the golden age of 'scientific' racism, after all. It's the age when people became very interested, indeed worried, about the ancient ancestry of modern nations; when patriots from every country were concerned to show that their people were 'pure,' whereas other countries had 'mixed' populations (which is apparently a bad thing).
Sources: I mentioned Gibbon (anywhere he talks about barbarians or moderns, and the chapters on the ancestry of the modern nations) and Macaulay (especially his essay on the history of Catholicism) and Middleton (probably his letters from Rome). A great modern discussion of the topic is in Vasiliev, A History of the Byzantine Empire, who treats it with regard to the Greeks but in a way that also applies to the bias against modern Italians.