@codydub03 Holy time traveler batman that was fast. I'm sorry for writing so much, but I have dropped tidbits of stuff you can read about. I can't give you references because there are too many... they are funto read, you'll find them on your own easily enough.... The short of it is that so-called terrorists and religious fanatics don't exist unless you count the psycho ones... in which case they're already thugs, serial killers and rapists, not religious fanatics, because they already did that beforehand... Peter of Sweden literally just had every village in his empire give up four or five of their deadbeats every winter (or similar) for his army, for example (they would promptly die and need replacements).
Now, the head honchos being fanatics is rather inaccurate. A reading of after-action reports, or what passed for them way back then, will paint a different story. In short, it's rare for the combatants to write about their foes as if they were bugs, the Mongols and their descendants being the obvious exceptions because they just bbqpwn everybody anyway. In other words, something like Baghdad Bob was uncommon, so you're actually trapped in the 11th-century equivalent of urban warfare and the high nobility equivalent is like ten feet in front of you, the chronicler, because they just stopped you from being destroyed by Mameluks or whatever.
There is a reason Kingdom of Heaven is considered an atrocity upon historical accuracy in movies. It's really difficult to think of anyone who considers anyone to genuinely be "infidels" and either doesn't have a good reason for it (because they're Mongols) or are themselves getting whooped and acting like poor losers.
The actual closest thing would be China invading a group of what were at the time coming to be Pan-Altaic peoples and forming a new Mongol Empire perhaps in the 1600s and 1700s, where, somewhat uncommonly for Chinese dynasties, the generic policy was simply relentless genocide irrespective of the number of infantry committed - probably because they remembered the Mongol Empire and Iirc the dynasty wasn't even Chinese and most of the soldiers were possibly themselves Muslims (the new pan-Altaic religion was not at all Islam, and Turks were not yet synonymous with being Islamic). As an aside, you might remember propaganda about Chinese Muslim (actually Turk) "terrorists"... now that you've heard about concentration camps, maybe the reason why is kinda obvious in hindsight. Had things been slightly different, the propaganda would paint Tibeten revolutionaries as Muslim terrorists and we would have never heard of Saddam Hussein, instead we'd find the weekend paper would have articles about the devilishly evil Dalai Llama and his suicide bombers.
Perhaps the closest thing might really be some of the northern European crusades, but given the propensity for most belligerents to both be Christians in the cascading wars that followed (the extermination of most Teutons by Poland in a single battle being the silliest example) or often predating the crusades, again, nobody is massacring just because they could. The only massacring just because they could with quasi-religious overtones is probably the American expansion west, and even that was orchestrated only by a tiny sliver of the population - 1000 army soldiers VS 500 Nez Perce when the Civil war just happened, for example - and even then, more Indians probably died from diseases brought by missionaries who set up missions than died from warfare.
I could go on and on, but basically villages just spontaneously going up in flames because their neighbors just thought to do so pretty much doesn't happen. A clue is in your genetics, if you are white. Your physical traits are mostly recessive. Aggressive hegemonizing swarms of pink goo, mathematically, cannot preserve recessive traits, and they don't give them the chance to get generated with much frequency. The only people who have been genuinely mongrel-ified or genetically replaced are maybe the Germans and the British... and the Angles, Saxons, Swedes, HRE and so on infantry were less interested in mindless religious massacre and more interested in not starving because one or another Wallenstein outsmarted them yet again, the defenestrations were associated with massive property exchange, etcetc... people don't just start killing each other for no reason, and if they claim to do so for religious reasons, there are almost always mitigating circumstances, that is to say, they were already born as serial killers.
Well anyway, the point is a good trip to an academic library would cause you to leave with so many books you could read one a day and still get nowhere and have your mind shattered every single day, which could be nothing but fun. As an aside, there is actually something like what you're talking about that does happen incredibly rarely, such as the occasional genocide by an Afghan king of Afghan pagans... but again, it's the overwhelming exception to the rule. If, however, you adopt a notion of meta-belief systems, then a type of null belief system can quickly give rise to just what you're talking about and literally every country in Southeast Asia today has just what you're talking about, but Rohingya the victims most certainly are not... again it is sadly niche knowledge. A non-violent example of a null belief system giving rise to this behavior is advocates of the notion that an Age of Enlightenment ever existed adding to the dependency chain of contemporary technologies early gear systems, early digital computers, quaternions and octonions and so on... even though those things originate from the 1400s, 30-years war and 1200s, respectively, and represent technological dead-ends because the inventors sometimes died horribly in conflicts of the nature the enlightened thinkers would themselves trigger the world over...and so you have Westerners taking credit for inventions that are Western but do not lie in their heritage, ergo kill all the dumb sand apes BECAUSE we have better tech... which is simply a religious belief, albeit one lacking mystique, so that would fit just what you're talking about.
EDIT Ah, I just thought of something! Peasants didn't do the INITIAL mass killings in the 30 years war... Burghers did it. Well, they're virtually middle class. Waddya know, but the black people I saw keeled-over in the streets of Philadelphia were most certainly not of the constitution to engage is physical activities like looting... The looters from last night are obviously middle class... What a surprise, hundreds of years later and the story is the same: Rich Burghers beat up Rich Burghers and blame it on brainwashed peasantry.