The motivation can be explained
This article about that JFK video game is interesting and well-worth reading, but it points to two things I think are misconceptions you'll find a lot with people who don't think that Oswald could have acted alone. He addresses the "single bullet" theory, but doesn't acknowledge that most people think that the "single bullet" theory is just that--the theory that Oswald got off one shot that made all those holes. Once you explain that the "single bullet" theory is the theory that Oswald got off three shots, two that hit, most people see things very differently. The debate is about whether three shots were fired or four, and the belief is that if it were four then Oswald couldn't have acted alone.
This writer doesn't acknowledge the other major argument that sways a lot of people to the side of the conspiracy theorists--that Oswald himself just couldn't have done it. A lot of conspiracy theorists dance around this one, because arguing motivation is just hard as hell to do. But it does sway a lot of people. We want to believe that a lone madman couldn't just do this for no real good reason. It has to be a politically motivated conspiracy, or we have to face the fact that random violence is possible.
Considering that a few years after that, a madman went into the UT Tower and managed to pull off 50 sniper hits, some of which are much harder to picture than the one than the Kennedy assassination, I am surprised to this day that people have trouble understanding that this sort of shit happens. Charles Whitman and Lee Harvey Oswald had nearly identical motivations--military trained losers who finally flipped shit one day and decided to punish others for doing better than they could do. I cannot for the life of me think why people think that Presidents, even young and charming Presidents, are somehow exempt from the attentions of lone nuts with guns who prey on everyone else.