We all see that he was gunned down at the Jones Beach Causeway, we all find out that Carlo Rizzi, his brother-in-law married to his sister Connie, tipped the Tataggia family off after Carlo gave another beating to Connie - Santino flew into a rage when he heard, and raced to beat the shit out of - and possibly murder - Carlo. (Truth is, according to the book, Santino didn't really know how he was gonna handle it - it depended on how bad Connie's condition was. But he wanted this straightened out once and for all, and who can blame him? Some of us have sisters, and we don't want to see them hurt by their husbands.)

What we don't know is that this all started not so much with Carlo beating up Connie, although that was the catalyst that put it into motion. An informer for the Tatagglia Family saw Santino (Sonny) beat up Carlo at Carlo's betting business in Manhattan after Sonny found out about Connie getting beat up by Carlo. It wasn't Carlo who betrayed the Corleone Family (at the time) - it was the informer who saw it, and ran to a telephone to report it. But by the time the informer got to the Top Level of the Tatagglia organization, it was too late - Sonny was already back at the mall on Long Beach, safe.

This was war-time between the Corleones and the Tatagglias, and if they had spotted or caught Sonny in the city, Sonny would have been popped. After the Tatagglias found out from the informer that Sonny threatened to kill Carlo if Carlo beat up Connie again, they knew they had to talk to Carlo, to play against Carlo's emotions, his humiliation not only against getting dehumanized by Sonny in the street (Carlo did not fight back), but the humiliation he felt at his position within the Corleone Family. He wasn't getting the respect he felt he deserved now that he was married to the Corleone daughter - so the Tatagglia's played it up, offered Carlo a "better life" away from the Corleone's. Carlo was dumb enough to think they were telling the truth, so Carlo set it all up. He beat up Connie, he went to his betting business in Manhattan, and made a phone call. He knew that Connie would call Sonny, and he also knew Sonny would come for him.

...but if it hadn't been for the informer, Carlo would not have been a catalyst to the death of Santino Corleone. Does that mean that he didn't deserve to die by the hand of Michael Corleone, years later? I say - he deserved to die. Hell yeah.

But your heart really goes out to Santino, if you read the passage from the book - the moment before he gets shot to smithereens:

And then in the fraction of a second before anything actually happened, Santino Corleone knew he was a dead man. And in that moment his mind was lucid, drained of all violence, as if the hidden fear finally real and present had purified him.

Truth is, when Connie made the call to the Corleone mall, she meant to speak with Tom Hagen, not Sonny. She knew Sonny would kill Carlo, and she didn't want that. She only wanted to be picked up and brought home. But, she got Sonny on the phone instead.

And the rest is history.