i wanted a shoot but i did not. anyone who knows the way to kill a shark?
Back to the original question, regardless if this happened or not, but just for your future reference;
Leave the shark in peace and DO NOT shot him unless shooting him would be a last resort to save your life or your buddies life.
Remember everytime you spearfish (in areas where there are sharks), you are probably entering and invading a shark territory, and competing with their natural source of food. Yet still shark attacks on spearos are extremly rare, which simply shows that when you see sharks, they aren't there to attack you, but they are simply in their territory and you are the parasite there....
How useful is a speargun in deterring a shark that is determined to "consume you" ? I guess very useless... If a great white shark set you as his prey, then you got no chance of escaping him, regardless if you were swimming with a speargun or powerhead. Not so long from now, a tragic incident happened in South Africa, where two guys were spearing there, and a GW stalked one of the spearos, grabbed him and completly consumed him. His buddy even shot at the GW, but that was just barely enough for the shark to notice a spear in him... The same shark was even seen couple of days later with the spear and speargun hanging from him....
In brief, if you are one of the few very lucky divers in the med sea to have that privilege of seeing one of those beautiful great white shark, then you should feel really happy and proud that it happened, cause you might never see a GW in your lifetime again (in the med sea). And simply, never risk your life or the life of your buddies with you by shooting such great creature simply cause he pissed you off by taking his preys (which you originally shot), and ONLY shot him if that's your last resort to save your life....
On a more personal note: happened to me couple of weeks ago, and returning from a long swim (1.5km from open sea) in one of the most beautiful areas in the persian gulf (natural reserve in southern Qatar), and when I was like 200m from the shore in like 7-9m of coral reefs water, just like 30mins before the sunset time, i see this huge (2.3m +) and thick bull shark beneath me just checking the reefs in the bottom of the seabed, and btw, those are extremly rare here and the chance of encountering them are very very slim... I just felt very happy but kind of anxious tooespecially that I was swimming alone (my speargun wasn't even loaded at that time too, nor did I stop and load it). I simply followed him calmly from the surface for more than 20m and then it disappeared in the blue. Shot him? That would have been probably the most stupid thing I could have done and might have costed my life.... I all the time swim back in that area and keep that encounter in the back of my mind....
My 0.002 euro cents
Best regards