Is Sonny really responsible for the hit in his father? By letting "someone outside the family know what he is thinking", did he give The Turk the idea to kill the Don, or would the Turk done the same thing simply b/c The Don turned him down?

Yes, he is.

No I don't think as a result of Santino speaking out of order got the Don shot. I do believe that it was a help in towards getting Santino not to overeact when the shooting happend. Remember the Turk said to Tom "Sonny was hot for my deal" this was a way for their not to be an all out war. But c'mon you think Santino was gonna let the Turk get away with that? Forget about it! Nothing was gonna stop him

Yes, it most certainly is the reason why the Don was shot. Read the book. It's mentioned that Sollozzo figured that since there was a split inside the family, he could take out the don and then get his way with the deal.

Sonny's indiscretion certainly encouraged Sollozzo to think that there was a split in the family that he could exploit. But I stop short of saying that Sollozzo wouldn't have tried to kill the Don if Sonny had been silent. Sollozzo was intent on getting into the drugs business. He calculated that he could kill the Don and make a deal with Tom and Sonny for the remainder of the Don's political assets. He probably figured that Sonny would try to kill him in revenge. But, with the Don dead, much of the Corleone Family's strength would have disappeared. Sollozzo figured that if he was careful and survived, over time Sonny and Tom would see that there was no percentage in holding a grudge, and that they should make a deal with him.

In some ways yes, because if you take out the Don, his oldest son (Sonny) would have to take over, and he was interested in the deal. He figured since Luca was gone Sonny might not go after him. (If he could kill a man like Luca, who was next?)Or it could have just been the icing on the cake.

Santino is completely responsible for the hit on his father because when he spoke out he let Sollozzo know that he wanted the deal. When Sollozzo hits the Don, he snatches Hagen to try and make a deal with sonny, he even mentions to Hagen that he knew sonny was 'hot for his deal'. I don't think that Sollozzo would have risked everything if he did not think that he could make a deal with sonny. That was the key.

I think that believing that Sonny would eventually make a deal was the big flaw in Sollozzo's thinking. Because even if the hotheaded Sonny had been able to play it cool (which even Michael himself wouldn't have done, after all he was the one who shot the Turk), Sonny would -- in true Sicilian fashion -- have sought revenge for his father's death, sooner or later. Deal or no deal. I believe that it was Barzini all along, as the Godfather said. Barzini must have advised Sollozzo to go after Don Corleone, knowing that eventually Sollozzo would be expendable. The Turk was an outsider anyway. Arguably, Barzini may have planned to have Solozzo killed eventually anyway, or have him move back to the Mediterranean to keep an eye on that end of the drug trade.

Sure. The Trilogy is full of examples of people taking advantage of others' ambitions, grudges, etc. Put another way: Neither Barzini nor the other Dons would have tried to assassinate Don Corleone by themselves. But when Sollozzo came along, willing to take the risk in order to establish his business and improve his standing, Barzini had nothing to lose by encouraging him.