The "Red Light, Green Light" massacre scene sets the tone of the entire series.
After Player 111 and a few staff members are executed for cheating, their bodies are hung up for display the next day for the remaining players to see.
The marbles challenge. Not only did it kill off MANY major characters, but it was arguably one of the most heart-wrenching scenes of the series, as it's clear to see that the main characters aren't in a safe place anymore. Perhaps the biggest surprise was at the beginning- players are asked to form pairs under the assumption that they're teaming up like in the previous game, resulting in them choosing their best friends, spouses or the people they trust most. They only realize that they're playing against those people, and at least one of them will have to die, when it's too late.
Sang-woo goes from logical and pragmatic to utterly ruthless when he cons Ali out of his marbles. Ali realizes that he's been had just before he's executed.
Sae-byeok is mortally wounded, and Gi-hun is banging on the door begging for medical help. The door opens and some guards walk in... with a coffin. We then see that Sang-woo killed Sae-byeok.
The final game. Gi-hun and Sang-woo play the titular Squid Game, but quickly abandon playing it for the most part. Instead they end up brutally punching, strangling and stabbing each other over and over as blood flows copiously. It ends with Sang-woo's suicide upon losing. Many viewers perhaps expected the two (former) friends to revolt against the organization or trigger clause 3, but it all ends in tears and a lot of blood.
The reveal that The Front Man is actually Jun-ho's brother, who won the game in 2015 and vanished.
The Front Man shooting Jun-ho, causing him to tumble off a cliff.
The reveal of who the mastermind behind the games is. Few things during the series really hint at Il-Nam being more than he seems. There are some subtle hints, like being aware of Gi-hun cheating him in the marbles game despite his dementia and his extensive knowledge on the games played. However, it is not uncommon for people with Alzheimer's to have lucid moments when the disease has not yet fully matured, while his knowledge could simply be chalked up to him having played those children games extensively in his youth days. What further throws people off is that unlike the Alzheimer's, his brain tumor and physically declining state as a result of it are not faked. By the time he reveals himself to Gi-hun after the games, he's already on death's door because of it.
