There's certain way you're supposed to act in court, and this isn't it. Hell, this really isn't it. Even if half the kingdom has risen up in open rebellion against your pompous behind and you're getting whupped all over the Riverlands, you don't publicly debase your adversary's sister and the woman you're supposed to marry, no less in the Throne Room, in front of braying crowds. Let's forget the fact that Sansa Stark is a noblewoman of one of Westeros's great houses for a moment. Regardless of her name, she's just a young woman, and no young woman deserves to be treated like that. It's bad enough that he'd verbally embarrass and aim a crossbow at her in public, but to have your guards hit her with sword-hilts and wind her with heavy-gauntleted fists is just so dang despicable, you're shocked even Joffrey could do it. Well, maybe not. But still, imagine what could've happened if Tyrion hadn't shown up and restored sanity. Before that point, the guards were in the middle of stripping her while Joffrey said she needs to scream louder. God knows what he had planned for her, but it sure as hell wouldn't have been good.