Just shut Picasso up in a room away from the strangers. Keeping him away from the strangers will keep him from getting upset, and will be better for everyone's nerves. I do want to warn you that the worst way to try to stop aggressive behavior in cats is with aggression and punishment. Punishment of any sort simply makes the behavior worse. I doubt it is anything you did or anything that happened to Picasso while he was a kitten.
Some cats are simply high strung and aggressive.
Let me know if you have any more questions.