I also don't believe he acted alone. At some level, at least, you have to believe in Mafia involvement. Who pulled Ruby's chain to get him to kill Oswald? More than likely, it was the Mob.
Where Johnson, the Warren Commission and the federal government get involved is in the cover-up. Johnson said near his death he believed Castro was to blame and hinted he needed to shove it under the rug to prevent World War III. It could be a lie but it might have also been the truth. There's no doubt, however, that the government did cover this up as only they can.
What we will probably never know is where the seed of the assassination germanated. Did somebody in the government approach the mob? Did Castro approach the mob? Did Russia urge Castro? Did the CIA urge Johnson? Did the CIA urge the Mob and then lie to Johnson so he would cover it up? Nobody can really say and practically anyone who could is now dead.
Personally, I believe Oswald was a pawn who knew what he was being told to do but his handlers knew better than to trust this one man to do the killing. I think there were other shooters who got off the fatal head shot. Oswald panicked and ran. Perhaps he had no choice.
Some argue that, by altering the motorcade route was proof the government had already put in the fix. Maybe so. But some of it, I think, was the innocence of the time. Like 9-11, people only realized after the fact that such a deed was even possible. The planners were able to get away with a lot of things simply because nobody realized how exposed to danger the President truly was. So a lot of sloppiness happens because nobody would take seriously that a president would be shot from a distance in an American city. All the previous assassins worked from up close.
I'm not ready to believe Johnson was guilty of anything more than the cover-up. However, I could be persuaded if somebody wants to put together an iron-clad case. Whoever does has got to be able to tie Johnson back to the only two people we are certain were involved - Oswald and Ruby. Anything less is just speculation.