He was manipulated and got caught. He tried to play with the big boys.

Marina Nikolayevna was originally living in St Petersburg, Russia. However, because of some sort of scandal involving a foreign visitor, it was best she moved to Minsk with her uncle who worked with the KGB. It was there she met Lee Oswald.

She was trained as a pharmacist. However there were some questions about how she came to meet Oswald. At the time, the Soviets were using young women like Marina to romance and marry Americans and other western visitors. They would then return to the husband's home country where they would get a divorce and then be free to travel about freely. 

The US government was aware of this practice and began watching them closely and admitting them into the country with the agreement that they could not receive a divorce. This could well be the source of friction and unhappiness in the Oswald home.  When Lee and Marina Oswald returned to the US there was no restriction or delays in the paperwork required. In fact, Lee Oswald got a loan from the government to cover the cost of returning to his home.

The couple became part of a Russian speaking group and met George de Mohrenschildt, a sophisticated, well off Russian ex-patriot who sort of took them under his wing. They were sort of an odd couple. On one hand, the man who hobnobbed with the rich and powerful and you young Oswald's who had very little. He helped them find better accommodations, and helped find work for Lee. They did not know he came into their lives under the direction of the CIA.

They also met Ruth Paine, who invited Marina into her home and allowed Lee to stay their on weekends when he got the job at the Texas School Book Depository. She said it was to help her learn the Russian language, which she was studying. On one hand she was more interested in a relationship with Marina and on the other, she too had CIA connections.

It was Ruth who recommended the Lee go to the book depository to look for work because her neighbor, Buell Frasier's mother heard there were openings. Mrs Frazier denies ever knowing about any jobs there.

Marina Oswald was crying for the what happened to the president and for Mrs Kennedy when the FBI showed up at her door looking for Lee. At the time she thought he had been the one who took the shot at General Walker, and she accepted that her husband had killed the president.

Marina and Lee's mother Marguerite went to the Dallas Police station where they were shown the rifle retrieved from the sixth floor of the book depository. She could not positively identify it and that was noted. When she was permitted to see Lee she said he was calm "by his eyes I could tell that he was afraid. He said goodbye to me with his eyes. I knew that."

Her initial reaction was that Lee could not have been the shooter, that he admired the president.  She was soon taken into "protective custody" where it was explained to her that unless she went along with the official story, her children would be taken from her and she would be deported back to Russia.  She made the same choice most caring mothers would.

At the Warren Commission she said she believe the evidence that her husband was the lone shooter, but over the years she became less convinced. The young widow in a strange land gave several interviews and tried to acclimate herself to the new reality.

She eventually married a man named Kenneth Porter who does his best to politely but firmly protect Marina from reporters, researchers, and the merely curious.

Marina and Lee: The Tormented Love and Fatal Obsession Behind Lee Harvey Oswald's Assassination of John F. Kennedy
by Priscilla Johnson McMillan

McMillan came to the story with a unique knowledge of the two main characters. In the 1950s, she worked for Kennedy and had known him well for a time. Later, working in Moscow as a journalist, she interviewed Lee Harvey Oswald during his attempt to defect to the Soviet Union. When she heard his name again on November 22, 1963, she said, “My God! I know that boy!”
 
Marina and Lee was written with the complete and exclusive cooperation of Oswald’s Russian-born wife, Marina Prusakova, whom McMillan debriefed for seven months in the immediate aftermath of the President’s assassination and her husband’s nationally televised execution at the hands of Jack Ruby. The truth is far more compelling, and unsettling, than the most imaginative conspiracy theory. Marina and Lee is a human drama that is outrageous, heartbreaking, tragic, fascinating—and real.