I'm not sure which is more shocking: the Red Wedding itself, or the fact that it's based on not one but two things that actually happened. No weddings, as far as I can track down, but two violations of the real-life version of "guest right." Scottish politics was a bloody business. The Black Dinner
Here's where the "eat a nice dinner and then get brutally killed, oops" thing comes from.
In 1440, the leading families of Scotland were sick of the Black Douglases. This lowland clan had gained a lot of influence over Scotland, with their leader, the Earl of Douglas, acting as regent for the ten-year-old King James II and so basically dictating how the country would be run. Not acceptable, at least in the eyes of all these other people who wanted to dictate how the country was run.
The Earl of Douglas died, of completely normal and natural causes, and his sixteen-year-old son William took over the title. A very Game of Thrones-esque power struggle ensued, as first one family had James, then another kidnapped him, then those two families united against a third threat, and on and on.
In November, 1440, these struggling lords invited William and his little brother David to Edinburgh castle to eat dinner with the young king. It was a dinner of "reconciliation", showing how totally fine everything was after the last chaotic few months, and the too-trusting William showed up without a worry. Supposedly, he and James got along really well, and everything was going great, until James' men threw a large black bull's head down on the table in front of William. The bull's head was a symbol of death, the color symbolised the Black Douglases, and the resulting "we're going to murder you" message was pretty clear.
The young King James was not happy, but his advisors ignored him. While James pleaded for them to stop, the two young Black Douglases were beheaded.
The Massacre of Glencoe
This story is a little longer, but it's where the idea of "breaking guest-right" comes in.
After years of political upheaval, England and Scotland's King James VII was in exile, and the countries had two new monarchs, William and Mary. There was a big uprising in the Scottish highlands to try and return James to the throne, but the rebels were defeated, and they were all offered a pardon, if they took an oath of allegiance to William and Mary by January 1st 1692.
This was a bit of a problem, because the rebels had already sworn an oath to James, and since he was still alive, they couldn't break it. So the rebel leaders sent a message to James in exile in France, asking him for permission to swear this oath to William too. James wasn't a big fan of this plan. He didn't want his supporters to get executed, but he thought he was on the verge of returning to Britain to take his throne back, so he couldn't have the rebels now swearing allegiance to his enemies. He dithered about, unsure what to do, until late December, when he sent orders saying, basically, 'Yeah, okay, if you must.'
The message didn't arrive to Clan MacDonald until December 28th, leaving Alastair Maclain, 12th Chief of Glencoe, only three days to deliver the oath to the correct authorities. He went to the nearby Fort William to ask the governor there to administer the oath, since time was so tight, but the governor said he wasn't authorised to do that. Instead, he gave Maclain a signed letter saying he'd at least tried to make the oath before the deadline, and sent him on his way to Inveraray, to make the oath in person.
It took three days for Alastair to get to Inveraray. Then he had to wait another three days for the only man authorised to take the oath to arrive. But the oath was made, if a few days late, and Maclain went home, reassured that everything would be fine.
A couple of weeks later, 120 soldiers stopped by at Glencoe, saying they needed somewhere to stay, as the nearby fort was full. The MacDonalds welcomed them, as was expected in the Highlands, and the soldiers stayed for two weeks, until their leader, a man called Captain Drummond, arrived. He played cards with the MacDonalds, and then wished them goodnight and went to bed, making plans to talk with them more in the morning. Then he ordered his men to kill everyone in Glencoe under the age of 70.
Alastair Maclain was dragged from his bed and killed, along with anyone else who didn't react fast enough. 38 men were murdered by Drummond's men, and another 40 women and children died out in the cold after their homes were burned to the ground.
This attack was considered "murder under trust" -- basically, the same as the violation of guestright in the Game of Thrones universe -- but any attempts to prosecute anyone were ruined by the fact that the orders had ultimately been signed by King William. And how could the king be wrong? Captain Campbell was imprisoned in Edinburgh Castle for a few days, supposedly for scheming with rebels himself, but otherwise, nothing was done. And so, to this day, Clachaig Inn in Glencoe has a sign on the door denying entry to Campbells. Better to be safe than sorry.