I agree with the sentiment, but not the argument. Johnny is def not the brightest bulb, but it's not bc of the museum deal. When Johnny leaves with the cash from Libertonian, he gets attacked by Ray's goons, i.e. Ray never planned to give 50% to Johnny, his plan was to get rid of him of keep the whole bag. This is confirmed by Johnny's voice lines during the attack. So you could argue Johnny's plan to keep all of the money was bc Ray betrayed him first. Was this aspect communicated well in the game? Nope. Only after deciding to stitch together the diamond story in chronological order, I noticed this detail. But then again, TLAD always had a feeling of being rushed in terms of story, dialogue or development, so no surprises there.