Krusty could be quite the hypocrite: he could steal the jokes of better comedians, then complain if someone else quoted one of "his" jokes.
And that wasn't the worst of it: to use a tired saying, Krusty could dish it out, but could not take it. He might rant at times about how he wasn't respected enough by anyone, whether it was the network executives, the media, fellow celebrities, his fickle fans, or the unlucky people who happened to work for him, but he never treated anyone else as though they had feelings or dignity.
Poor Sideshow Bob had learned this the hard way, after Krusty punched him in the face on live television. All Bob had done was smear one pie in his face. After getting punched, the shock had left Bob stunned on the floor until commercial break, out of sight from the cameras, but in full sight of the children in the live audience. The little ones would probably start hitting friends or siblings, just because they saw Krusty do it.
To Bob's relief, the blow hadn't broken his nose, but he did have a conspicuous bruise. As he put ice on it, Bob couldn't help but dwell on the unfairness of it could hit Bob with as many banana cream pies as he liked, and the impact of getting hit by a pie was surprisingly painful. Sometimes, Bob would get pieces of pie crust in his eyes, which was especially excruciating. But when Bob very lightly smeared Krusty with just one pie...BASH! And the day before, Krusty had used Bob as a human mop, another reason for Bob to curse his hair. He'd considered just shaving it off, but it turned out that his contract wouldn't allow that. His contract forbade him to do numerous things, as if Krusty knew Bob would become dissatisfied, to put it lightly.
Bob stewed on this until the ice melted in his hand. Everything was unfair. When Bob officially signed his contract to be on the show, he'd been too stagestruck to read it carefully. Among other things, the contract stated that Bob couldn't sue Krusty if he ever got hurt by the clown's ill temper or poor sense of humour. After thinking about his carelessness for a while, Bob resisted the urge to give himself another bruise.
It was a good thing that Krusty would endorse nearly anything; Bob found himself a nice dartboard shaped like Krusty's face, along with a life-sized Krusty punching bag. Through these products, Bob could attack Krusty's grotesque, pasty face whenever he liked.