I know exactly which moment you're talking about. I'd been spoiled for how to spare that character; I wonder if I'd have fallen into the same trap otherwise.

It wasn't guilt-related, but Undertale did absolutely terrify me at one point: when Flowey crashes the game at the final battle. It felt like something he was doing to attack me, the player, rather than the character I was playing; I wasn't prepared for it at all!