There was something that Bob felt he would regret for the rest of his life, and it wasn't even one of his evil deeds.
It happened about four years before Bob snapped...
"Wasn't that a great Itchy and Scratchy cartoon, kids?" Krusty said cheerfully to his audience. "We've got another one coming right up, but first, I've got a hankering for some pork products!"
That was Sideshow Bob's cue to put on a chef's hat and wheel out the barbecue covered in pork products.
"Mmmm, look!" said Krusty. "Plump, succulent sausage, honey-smoked bacon, and glistening, sizzling-"
Suddenly, Krusty gasped, clutched his chest, and dropped to the floor.
Oh, Herschel Krustofsky, Bob thought with contempt, faking a heart attack is a new low, even for you.
The cameramen and the kids in the audience were laughing uproariously, believing, like Bob, that it was just a joke. But when Krusty didn't immediately get back up, Bob finally became a tiny bit concerned. It was very unlike Krusty to hold up a joke for this long, especially when there were delicious pork products nearby.
Bob had to check on Krusty without breaking character (his contract forbade him from communicating with anything other than his slide whistle during a show, under any circumstances, for as long as he was Krusty's sidekick). He ran up to Krusty's inert form, poking at him with his large foot and making quizzical noises through that stupid wind instrument, as the children kept laughing.
Oh dear! Bob thought with increasing dread. This was almost as if God was punishing Krusty for his non-kosher diet (Bob was one of the only people at the time who knew that Krusty was Jewish).
Bob almost grabbed a piece of bacon off the grill to hold under Krusty's nose in an attempt to revive him, but thought better of it. He gave the cameramen Krusty's "Go to commercial!" gesture and ran to call 911 on the backstage phone, inwardly cursing the bystander effect that seemed to have plagued everyone else.
Bob later wished that he'd been affected with it, too.