As for my theory, I believe that it was the mafia who killed JFK. They had the motive since they helped him through back channels to rig elections in WV and IL (at the behest of the old man) to get him in the White House only to find JFK and Bobby persecuting them later.
A couple of years ago it came out the Kennedys were still running a Castro assasination effort out of the AG's office where officially they were supposed to have disbanded efforts because of promises made to the Soviets. These assassination teams came from seedy parts of the world, thugs, militants, etc.. They just used the setup to kill JFK instead. So what the mafia did was get Oswald to shoot from one angle while discreetly getting another shooter in position. Afterwards, it would be hard to implicate these assassination teams since they came from their own illegal program. One conspiracy buff, and I cant recall his name, believes he tracked the assassins back to southern Europe. David Horowitz also shares my opinion.
I also believe that Oswald could have made the shots from his position. Its not imposssible as some claim and has been proven in tests.