I must have pissed off the wrong people because whenever I try to talk to Johnson Nash, everyone in the casino opens up on me. Did I do something wrong? Or have I not done something I need to first? Please help with this. I even restarted the game and avoided killing a lot of people only to have the same result.

Haha. The exact same thing happened to me.

I think what occurred was that I was dressed as a member of the powder gangers when I entered the casino.

The town of Prim has had a rough time with Powder Gangers, and won't appreciate seeing one in close proximity.

Try to change clothes.