What is the logic behind riots? What do they want to achieve( more specifically, the ones in Greece right now)?

I mean, the government already are prosecuting the police responsible for the killing and apologized. What else do they expect the government to do? Will it just end up like the ones in France several years ago? Nothing productive really came from it. Only destruction of property, which undermines the rioter's supposed want of reform because it costs money and time to recover, putting reform in the back seat.