Well, sacks of cities were very hard to prevent, because loot was the main reward of soldiers much more than their actual pay. In terms of a purpose, it was all part of Tywin’s plan to make sure that “no man could doubt that we had forsaken House Targaryen forever.”
Ned, on the other hand, had no such plan because he had no such need. Now, if Ned beats Tywin to King’s Landing, chances are the gates are not going to be opened to him, because Pycelle is only Tywin’s bbf. So there’s going to be a fight for the gates, and that’s what historically led to sacks. Historically, it was understood that a city that fought rather than surrendered would be sacked, because of the high mortality rates for assaulting armies.
However, the odds weren’t 100% for a sack – a good commander with good subordinates could keep their men in check, and Ned was pretty good. My guess is that he would have emphasized the need to take the Red Keep once they had gotten inside the walls, offering great rewards for the men who took the keep and threatening harsh punishment for anyone who deserted to loot before the Keep was taken, making sure that (rather than sending the common soldiers into the breach as cannon-fodder) officers were in the front lines to guide the men through the breach and towards the castle, etc.
There would probably still have been violence in the city, there’s no way to avoid that, but on a much smaller scale compared to the destruction of OTL. Certainly, I think Ned would have kept Elia and her children alive, probably to try to get an exchange of prisoners for his sister.
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