It’s probably a lack of knowledge, but the 30 years war was extremely bloody considering how little it actually accomplished. For some reason the majority of Europeans felt the need in intervene in a relatively obscure issue. When Spain discovered America, they had free reign to kill millions and wage war as a default diplomatic interaction. And Julius Caesar killed untold numbers of Gauls and Romans without any real reason beyond glory and exploitation of the weak. Yet I feel like during the Victorian era, you can’t really try to conquer large areas or conquer purely for imperialism alone. If for example France tried to conquer northern Italy again all Europe would explode in anger, and no one is EVER going to call themselves the aggressor. It’s always “we were mistreated, they attacked us, we have this long list of crimes they committed against us so we’re really just getting even.” Before people said “Yeah, I killed all those innocent civilians. Yeah, I’m a conquerer and I’m proud of it. I kill and repress in the name of my glorious dynasty/country, and I don’t care about the enemy’s welfare”. Is this just propaganda speaking, or is it true?
You're not entirely wrong, but are conflating a few things.
First of all, it's important to distinguish between different kinds of conflicts. One of the most brutal type of wars are what are termed "great power wars" — massive conflicts between powerful nations capable of mustering significant resources and armies for the struggle. A particular subset of great power wars are what I might call "great power coalition wars," which involve many great powers at war all at the same time. The Napoleonic Wars, World War I and World War II were particularly large great power conflicts, and it's true that no war on the scale of those (many powerful combatants, multi-year duration, continent wide theaters of operation) took place in between 1815 and 1914 in Europe. But there were plenty of more localized wars between major powers, including the Crimean War (1853-1856, involving France, Britain, Russia and the Ottoman Empire), the Franco-Austrian War or Second War of Italian Unification (1859, France, Piedmont/Italy and Austria), Austro-Prussian War (1866), the Franco-Prussian War (1870-1). After South America's independence wars concluded successfully in the mid-1820s, many of the newly independent states spent much of the succeeding century fighting each other.
There are other categories of war as well, of course. Civil wars can take a variety of forms, from state-sized entities fighting pitched battles (like the U.S. Civil War) to a complete collapse of civil peace — what Hobbes termed the "war of all against all." The mention of the U.S. Civil War is a good counterexample to the idea that this century was completely peaceful: up to 1 million total people died in that conflict, right in the middle of your century of peace. It's also easy for Westerners to forget the rest of the world, such as China's Taiping Rebellion, which overlapped with the U.S. Civil War and was an order of magnitude deadlier with as many as 20 to 30 million total dead. Europe also saw its own civil strife in this period, most notably in a series of uprisings around the continent following the year 1848; France saw the Paris Commune of 1871.
Finally, there are wars in which a great power attacks a lesser power, of which colonial wars are a subset. Certainly if you were a resident of Africa during the latter half of the 19th Century, you might not describe it as a period of peace! The Indian Rebellion of 1857 led to as many as 800,000 deaths. The United States' conquest of Native American tribes over most of the century was quite violent, especially as a share of populations living in the disputed areas.
(All this of course ignores one-sided internal state-directed violence, a category in which you might include repression of dissidents and protesters, and chattel slavery in the antebellum United States.)
What can we draw from all this? First of all, clearly the century between Napoleon and the First World War saw its fair share of battles, including wars between great powers, civil wars, and colonial conquests, in which many millions of people died. It's also true that there was nothing in Europe on the scale of the Napoleonic Wars or World Wars in this period. But we can complicate this assertion further by sub-dividing this 100 years in Europe into two parts: the "Concert of Europe," in which a conservative post-Napoleonic order prevented great power European wars between 1815 and 1848, and the considerably more violent post-1848 period after the Concert's collapse, which also saw the "Scramble for Africa" and other violent colonial wars in Africa and Asia.
So any statement about the 1815-1914 period being unusually peaceful requires so many qualifications as to both geography and degree that I'm not sure it's really a useful statement to make. If you want to speak more precisely and usefully, you can speak of the absence of great power coalition wars (or some other, better term) over this hundred years. Those are uniquely devastating and their absence is not nothing! But plenty of large-scale bloodshed took place over this time period.
Sources
I don't have a single source to recommend, as I'm speaking from my general reading concerning the period, and specific knowledge concerning France in the period. One starting point for Europe could be Richard J. Evans' The Pursuit of Power: Europe 1815-1914. I'm not an expert in Africa, Asia or South America during this time period but there are other experts here who can weigh in with reading suggestions for those areas, or answers delving more specifically into them.