Not that this would have mattered, but another flaw to "Don" Santino was that it wasn't too hard for his enemies to find him. For example, we all see at the end that Barzini knows Santino will visit his sister after he hears about the horrific beating Carlo give Connie. But another thing to keep in mind is that Sonny was addicted to Lucy Mancini (the maid at the wedding). He would often "hook up" with her. Even though Sonny always had bodyguards with him, his enemies could have hide inside Lucy's apartment and murdered him.

All and all, there were just to many flaws to Santino's charchter. Which is why clearly dies.