I believe it is explainable as a perfect storm of factors.
It's clear from Oswald's own writings and statements to Marina that he had an exalted view of himself. Yet he found himself a complete nobody in the USSR (to his great surprise) and a complete nobody upon his return to the US (to his great surprise), now working at a demeaning, dead-end, temporary, minimum-wage job in the TSBD. At every turn, his life had been failure upon failure.
I believe his last hope was what he expressed to Marina: He would be a hero to Castro and the Cuban people in the "Marxist utopia" of Cuba. All of his activities in the months preceding the assassination suggest a concerted effort to establish his Marxist credentials for presentation to the Cuban consulate in Mexico City. Alas, his trip to Mexico City was yet another failure.
To top it all off, his marriage was failing and Marina rebuffed his attempt at reconciliation. Had Marina given him another chance that Thursday evening, I dont't believe the assassination would have occurred. I believe the Thursday visit was a last-ditch attempt at a normal life, and Marina's rejection was the last straw. When Oswald saw the route of JFK's motorcade, and realized it would pass right in front of the building where he happened to be working, it must have seemed that Fate was speaking to him, but I believe it was Marina's rejection that sealed the deal.
It is not at all difficult to believe that he would have viewed the assassination of JFK as his ticket to Cuba or at least as validating his authenticity as a genuine Marxist revolutionary. My guess is that he expected the assassination to be the proverbial "suicide by cop" and was astounded to find himself surviving the lunch room encounter and boarding a bus outside the TSBD. Nothing in the immediate pre- or post-assassination events suggests serious, careful planning. Finding himself outside the TSBD must have seemed almost too good to be true, and I believe his objective was probably to get back to the Cuban consulate in MC.
His silence after his arrest demands an explanation. He was quite politically savvy, and having survived the assassination I believe his objective was to maximize the theatrical aspect. He realized that by denying everything he would ensure a lengthy, high-publicity trial in which his Marxist political genius (in his own mind, anyway) could be expressed for all the world to see. Hence his hope for John Abt to represent him.
I don't completely discount any possibiity of a conspiracy. If there was one, however, it would have related to events in Mexico City. Someone may have promised safe passage if he made it across the border or even safe haven in Cuba. Any such conspiracy would have been so small and ill-defined as to scarcely qualify as a conspiracy - certainly nothing that would satisfy the conspiracy theorists who demand an elaborate, high-level conspiracy involving numerous agencies, organizations and individuals. In every conspiracy scenario I've studied, Oswald is a cardboard figure who bears no resemblance to the real Oswald of history.
Oswald's "patsy" statement is surely the most misused statement in history. If he'd been suggesting he was an innocent patsy in someone else's conspiracy, he would've blabbed his head off to anyone who would listen, including the press. His actual statement that he was a patsy of the DPD, arrested only because he had lived in the USSR, does not even vaguely suggest an assassination conspiracy and fits perfectly with what I believe to have been his actual objective of turning the future trial into Marxist theater. I'd be fascinated - I'd love it - if the JFKA was the product of a massive conspiracy involving LBJ, the CIA, FBI, Army Intelligence, DPD, Mafia, et al., but I believe it's pure fantasy.