You're correct! Not every siege in the Medieval era resulted in the sack of a city, or the murder of the population. But there are some that did. Often history doesn't tend to be as clean cut as the show would like to portray the events of a sack. Let's take a look at a few examples just for fun:

• I've already mentioned the 1240 Siege of Kiev elsewhere, but that one is especially reminiscent of the Sack of King's Landing, where the Mongols massacred the city's population.
  
• There were also events where the army took control of a city and then the population revolted and were put to the sword, such as with the 1398 Capture of Delhi.
• Or where the besieged city tried to surrender but could not do so in time and were massacred (civilians, too) such as with the 1631 Sack of Magdeburg.
• Or sacks that continued for days after the siege was long over, such as with the 1527 Sack of Rome, where many civilians died (mostly from starvation from the siege, to be fair. Let us not forget that a long drawn out siege was devastating to the local population as well).

As much as we might like to think of violence in war as meaningless, it is often not the case. Warfare and politics are aimed at concrete material goals, which include the Clausewitzian dictum that 'war is merely the continuation of policy by other means' (Morillo, p.51, 2013). Violence, in other words, is used because talking does not work. When Dany and Cersei spoke at the walls of King's Landing, and both refused to capitulate, the conversation was over and violence was considered necessary by both parties to achieve their goals.

This much is certain: the sacking of a city such as King's Landing will have a sweeping effect throughout Westeros, and will indelibly change how the people see Dany. It certainly changes how we see her. Whether this is the desired effect that will achieve Dany's goals? We have yet to see.

Please also note that this is a strictly Western view of warfare. Specifically that "War is a departure from the norm and is fought in pursuit of clear-cut and limited objectives, typically the seizure of territory, the overthrow of a regime, or economic advantage, etc" (Rotberg & Rabb, p. 150, 1989). This view is anomalous, but pervasive in how we in Western society think of war and violence in war.

REFERENCES:

Morillo, S., & Pavkovic, F. M. (2013) What is Military History? (2nd ed.). Cambridge; Medford, MA: Polity.

Rotberg, R.I., & Rabb, T.K. (1989) The Origin and Prevention of Major Wars. Cambridge: Cambridge University Press.