Flint Marko didn't kill Uncle Ben. He was convinced by him not to go foward with the robbery. Carradine came out of the building, took the gun from Flint's hand and shot Uncle Ben. Marko tried to stop him, but got hit with the gun and left behind. He wanted to help, but he wouldn't do any good to his daughter in jail, so, he left. Eye witness mixed up the confusion and tough he killed Uncle Ben. The police arrested him and gave him a hard time. Marko's fragile mind split in an more evil personality, the Sandman per se, and a lighter one, the true Marko. Sandman only shows up in moments of great anger, like the Crane Fight, and not for long periods of time. Everything Marko spend on jail made him believe he killed Uncle Ben as a shield to justify the bad things he was experiencing.
Yeah, yeah, far-fetched, but at least it cleans the mess!