Al just confirmed that Michael pretty much fell in love with Kay when he met her and he still loved her even during his first marriage and that Michael loves her even after she aborted his male heir and they separated for a long time.
He reunites with Kay and marries her a year later, promising to make the Corleone family legitimate within five years. They have two children, Anthony and Mary. They are born within two years of each other, leading Michael to joke that Kay is "more Italian than Yankee".
By The Godfather Part II, Kay has decided that Michael is a cold, distant, inattentive husband and father, but only after an attempt is made on his life in the family home does she resolve to rebel against him. At first, Kay's rebellion is silent and private: she aborts the child she is carrying.
Michael escapes to Sicily and spends two years under Corleone ally Don Tommasino's (Corrado Gaipa) protection. Michael falls in love with and marries a young local woman named Apollonia Vitelli (Simonetta Stefanelli).
He then flees to Sicily, where he falls in love with Simonetta Stefanelli's Apollonia. Tragically, she's later killed in a car bomb planted by Michael's bodyguard Fabrizio (Angelo Infanti), who betrayed him to his enemies. While Michael gets revenge on those who ordered the hit, Fabrizio is never seen again.
If Michael had changed earlier, had he begun to open to Kay; Michael and Kay could have stayed married together, spending their retirement years in Sicily. However, even after the divorce and Kay remarrying, Michael was still devoted to Kay. Including Kay, who confessed to Michael that she still loved him.
An interesting side note is that there is no indication in the novel or the first and second movies that Michael or anyone ever tells Kay that Michael had been married in Sicily let alone how his first wife died.
"We trusted each other." And though they never dated, "I had a little mini crush one time," she admits. "He was performing 'Computer Blue' and he was oily and shirtless and for a moment I was like, 'He's really sexy,' but then I got myself together. We had great chemistry. But our romance was only onscreen."
Kay was more than just business. She was his connection to home, his humanity and his dreams. He had to be a better man with her...and Michael wanted to believe he was a better man.
During his stay in Sicily, Michael married a local girl named Apollonia Vitelli (Simonetta Stefanelli) and, after her death, he returned to America where he married Kay Adams (Diane Keaton). He and Kay went on to have two children, Anthony (Anthony Gounaris, James Gounaris, and Franc D'Ambrosio) and Mary.
Despite this, the two of them maintained a loving relationship in 40 years of marriage, and Vito never sought a mistress or ever found the need to strike his wife. Carmela diligently managed the household in times of both peace and trouble, knowing the schemes of her sons and the nature of husband's associates.
Enraged, Sonny speeds towards Connie's apartment in Hell's Kitchen ahead of his bodyguards. At the Long Beach Causeway toll plaza, Barzini's men trap Sonny and shoot him to death. During a meeting with the other crime family dons to establish peace, Vito realizes that Barzini masterminded Sonny's murder.
They would have been married for almost a year. She would have been pregnant with his child and would have learned how to drive. Sadly once she was killed, Michael was never the same after that.
Fredo later betrays Michael after being approached by Johnny Ola (Dominic Chianese), an associate of rival crime boss Hyman Roth (Lee Strasberg). Ola and Roth tell Fredo that Michael is being particularly difficult in business negotiations between Roth's organization and the Corleone family.
Yes, he did, at least according to the unfortunate and ill-conceived sequel The Godfather Part III. In The Godfather Part 3, why does Michael Corleone break down and confess to ordering the death of Fredo when he hates Fredo and wanted him dead and said that he was nothing to him and not a brother anymore?
Michael has become a true Corleone (embracing both Italian culture and mob affiliation), and his marriage to Apollonia signifies Michael's official acceptance of this change and what it means for his future. When Michael marries Apollonia, he is wedding himself not just to her, but to Sicily and his father's values.
The third and final film in the Godfather saga, The Godfather: Part III, set in 1979, explains that Hagen had died a few years earlier. His son, Andrew, is ordained to the priesthood, and is assigned to the Vatican on Michael's recommendation. While talking with Andrew, Michael remembers Hagen fondly as a great lawyer.
Michael was alerted to the plot when Fabrizio ran away, and he attempted to warn Apollonia. However, in starting the car, Vitelli triggered the car bomb, and she was killed in the explosion. Corleone would later send a hitman to kill Fabrizio in New York.
After her brother's debut concert, the assassin Mosca tries to kill Michael. One bullet grazes Michael's shoulder, but the other accidentally hits Mary in the chest, fatally wounding her. Michael is devastated by Mary's death, and screams in torment while cradling her dead body.
Of course Michael had good reasons to lie. The biggest was to protect Kay. What Kay did not know could not hurt her. Because Michael left her out of the Family business, nobody had reason to kidnap her or toture her to extract information.
