And I haven't gotten that far myself, but apparently in the postgame you can visit Guzma's childhood home and
it's heavily implied that his father abused him as a child. So basically, it's implied that Guzma grew up in an abusive home, ran away, tried the Island Challenge, failed it, had nowhere to go, and then started Team Skull so that other failures and misfits would have a place to belong to. This actually explains Team Skull's behavior very well; they have no grand schemes like the other villainous teams and are just trying to steal enough stuff to survive. The Aether Foundation are the true villains of the story.