Episode 6, as everyone said, was absolutely devastating. I genuinely gasped at the announcement that you'd be playing your partner with the loser being eliminated. It's something I should've expected but for whatever reason I didn't. And I then spent the entire episode cringing watching each individual game unfold. Ali's ending just about brought me to tears, he was much too trusting for that game.
The contrast of Gi-hun and Sang-woo's cheating. Gi-hun feeling riddled with guilt the entire time and Sang-woo being relatively stone faced knowing he did what he had to do. But it also makes you face some cold introspection because no matter how much you don't want to admit it, you'd have probably done the same thing in their shoes. I mean it's that or take a bullet to the brain.
I knew very early on that the cop's brother was involved in the games in some capacity, I just didn't know if he was a random soldier or whatever. Once the files showed he won I thought he could be the Front Man.
But 001...fuck. In the first few episodes I did have a passing thought that he could be involved. There were little things, he gleefully played the first game and seemed unphased by everyone dying. The voting system ensured he'd vote last. Him managing to be the only one to get to a safe place during the night battle and the carnage only stopping when he called for it. But I thought I was overthinking it. By the time episode 6 came around the thought never crossed my mind. Even his death not being shown didn't raise a red flag to me because Ali got popped off-camera too.