Intense episodes of violence and murder shown in Game of Thrones tend to take place in war zones, not in peaceful contexts (you will not notice the same kind of casual murder in King's Landing). Violence during war is not something particulary medieval; descriptions of the Syrian, or, to reflect on a recent commemoration, Rwandan civil wars should convince you that there are still many contexts in which “people murder people and just walk away like nothing happened.” On the other hand, it is true that violence in a peaceful context (something we would normally call “crime”) was rather more important in the Middle Ages, or rather in pre-modern societies, than it is nowadays in Western Europe or the US. Historical criminology is not exactly something easy to deal with, but it seems reasonably clear that homicide rates, for instance, have been declining in the past centuries. But once again, these homicide rates were the product of a particular culture of honour, of a strong sense of belonging to structures that you had to defend, and to a (relative) lack of efficient enforcement. The exact same ingredients can be found in countries like Honduras today, and most estimations of medieval crime rates would tend to give lower murder rates in 14th-century London than in 21th-century Tegucigalpa.

I would therefore say that your perspective is biased by a sense of modern “safety,” which in fact hides the persistence of violence in a wide array of different contexts. As for the credibility of this violence in a medieval context, well, there were certainly military conflicts in the Middle Ages, and you can find as many gruesome descriptions of murder as you want in various sources (French accounts of the Hundred Years' War come to my mind as a pertinent example). But the violence depicted in Game of Thrones is not specifically “medieval.” To put it otherwise, your question is basically “were medieval people particularly violent?” — my answer would be: “they were particularly violent in the same conditions that still tend to create violence in modern societies (and that already produced violence in Bronze Age societies, for that matter).”

Looks like it's time once again to quote my favorite passage from a history book ever, about the response to the murder of the Holy Roman Emperor by his nephew in 1308. This comes from The Habsburgs: Embodying Empire by Andrew Wheatcroft, page 35:

[The Habsburgs'] first response was to take revenge on all those who had been involved in the murder of Albert. The four murderers had fled, but enforcing the old Germanic tradition of 'blood guilt', Albert's sons and daughters exterminated the entire lineage of John's associates. On a single day, Leopold and his sister [Agnes](https://en.wikipedia.org/wiki/Agnes_of_Austria_(1281%E2%80%931364)) sat on a dais and watched as sixty-three blameless members in the household of Baron Salm were brought before them and beheaded one by one; the corpses were arranged in long rows before them. As a head was severed and the blood spurted, Agnes was recorded as crying exultantly, 'I am bathed in May dew.' It was her own retainers who stopped her from throttling the baby son of another of the conspirators. More than a thousand died in this welter of Habsburg vengeance.

I'm actually a little skeptical of the veracity of that particular story (Wheatcroft doesn't cite his source for it, I could easily believe that it's just exaggerated local legend). But the fact that Albert's murderer was forced to flee in the first place, and was given the epithet "Parricide", suggests to me that murderers, at least of important public figures, could not simply "walk away like nothing happened".

Gregory of Tours wrote the "History of the Franks". In it are discribed several stories, several of which are in the 'Game of Thrones' type situation. My favorite example is from Book III Chapter 18. King Lothar is asking Queen Clotid if she would rather her sons hair be cut (making them commoners and no longer "royalty") or if it would be better to kill the boys outright instead. The queen says she'd rather they dead than have their hair cut off. So King Lothar stabs his nephews in the armpit killing them both. This is after one of the boys begs for their life and the two Kings debate on whether to kill the last boy or not.

The Franks did some metal shit, some of which I would say were way more violent than stuff I've seen on GoT.

I'm surprised that no one has referenced the 100 years war. Barbara Tuchman in "A Distant Mirror" vividly describes the awfulness that was Northern France in the 1300's. Basically, you had mercenary groups (Mainly English, but also Bretonnian, Burgundian, et. al.) who plundered, robbed, and raped their way across much of Northern France. Additionally, the English themselves gave the Huns and the Mongols a run for their money when it came to murder and rapaciousness. Many second (and third) sons of upper class houses in England saw the campaigns across France as an opportunity to enrich themselves and took full advantage of it. The awfulness of the sack of Caen in 1346 which caused the deaths of over 2000 residents of the city was well described by Tuchman and others.

The English didn't have it all their own way, however, as the King of France (I believe that it was Philip VI) at the time authorized raids on the southern coast of England.

Did they drink as much wine? And why do they spread rushes (hay?) in the rooms? Was that really a thing?

Opening note: I haven't read the ASOIAF books in a long time, and I only watched 1.5 seasons of GOT before giving up. I'm running off of memory and some quick google fact-checking here, so I absolutely welcome rebuttals and corrections.

There's an art history project hosted on Tumblr called MedievalPOC that focuses on historical representation of people of colour, and the author did an article on the sociology of Westeros called "Things Were Just Like That Back Then".

The author poses that while GRRM places Westeros in a quasi-Medieval setting, the violence levels are actually more representative of colonial brutality because European colonialism and imperialism in the 18th century brought in a whole lot of elements of violence that previous centuries lacked: mass depopulations by disease, visible chattel slavery, accumulation of wealth by colonies, etc. I've noted this in my own research too, but Westeros is really intensely racist/sexist by modern standards for a society that's supposedly pre-Enlightenment, so comparing it to historical levels of violence in the Middle Ages gets really tricky when the kinds of violence being carried out in Westeros didn't exactly exist then.

It's also noted that Westeros' level of violence is unlike the historical Middle Ages because Westeros' violence is unsustainable, yet somehow has persisted on and on and on. You can't just butcher your workforce (or prospective workforce in the case of claiming territories) for decades and expect people to just sit and endure it instead of leaving/moving, especially not if you expect to feed and clothe and populate your armies for these non-stop wars. All this and yet slavery is apparently illegal in Westeros... and yet there's this ubiquitous decades-spanning power, particularly one that allows nations to take the most violent courses of action possible and have them carried out to the fullest without being nipped in the bud. Real populations just don't sustain that kind of extended, prolonged brutality without uprisings. Westeros tends to sacrifice local politics for the sake of its warring aristocrats continent-spanning politics, by which I mean there are somehow very few national/local distractions or oppositions or uprisings getting in the way of their ability to constantly be fucking with each others' thrones. Which is fine for telling a story, but it's a really laughable premise for a medieval setting where long-distance politics take a back seat to local politics given the ability to travel, communication, etc. It's something like 3000 miles between Dorne and The Wall –– that's about the width of the United States, as much as twice the distance from Edinburgh to Naples! International politics and warfare in the Middle Ages just didn't span like that, no way, there's a big reason why Westeros-level massacres and brutalities didn't happen in our history until advanced travel and communication made it possible.

I mean, if all of Westeros has a population of say, 40 million, and is roughly smaller than Canada (which has a modern population of 35mil), and soldiers make up like 1-2% of the population, then you have like 800,000 soldiers. Now divide those between different nations in Westeros... then I haven't the faintest idea how you can have, say, 75,000 soldiers killed in a single war that lasts only two years. That's the estimation I've heard most often for the death tolls in the War of the Five Kings, and 75k soldiers is basically a huge chunk of the armies across Westeros, especially when the American Revolutionary War took eight years to kill only 8000 in combat on the Eastern coast of the United States... I have no idea how you axe that many soldiers across a territory more than twice as large with much slower travel times.

I also don't get how the Lannisters can get away with cruelty to their people and rule under the idea that "peasants should fear their rulers more than they fear the enemy" and "smallfolk don't care about political games as long as it doesn't effect them" when something like 25-30k of their soldiers were killed and the people of King's Landing have been starving for months and all that happens is a riot of a few hundred people and violence in the city for a bit (which only hurts the people more.) IIRC Tyrion orders a curfew to stop the riots and anyone who breaks it is executed, and yet this works instead of prompting even more uprisings. And this is earlyish in the books –– it baffles me that they continue afterwards with relatively the same players at the helm.

TL;DR: I think there is just so much more precedent for this kind of thing happening in a colonial world post 1800s, not a medieval setting. Is there ANY international conflict in the 1400s-1500s that results in such massive death tolls and violence against the population in such a short period of time?