It is probably fitting to observe that (as shown in one photograph), the Carabinieri jeep at that point had had the windows smashed in by protesters using wood planks and metal pipes. Blood was found on the inside, and the carabiniere that did the shooting was probably wounded.
The driver had managed to get the vehicle stuck and separated from the rest of the police forces.
Notice also that the person that did the shooting is a carabiniere ausiliario, that's to say a draftee that elects to serve in the Carabinieri instead of the regular Army. This means that he had had very little former experience of crisis situations, riots and violence.
I can safely say that, if I had been in the same positionI would have done exactly the same, i.e. shoot. But he should not have been in that position, nor should anybody.
About Carlo Giuliani, it is difficult to say anything meaningful. I suspect that he did not expect to be shot. But this is the general impression of the whole Genova G8 fuckup: a lot of people went there thinking not "demonstration" but "party".
Of course, we do stupid things when we are young. And we do stupid things when in the company of masses of excited people.
Still, threatening someone who is terrified, trapped in a corner and armed is not a very smart move - regardless of your political preferences.
Giuliani's father was impressively dignified on TV, although visibly in pain for what had happened to his son. He said that Carlo "could not tolerate injustice, under any form". Grieving parents don't need to be logical - still it beats me how you go from disgust for injustice to throwing fire extinguishers on cops just like you.