When the war ended, the Army began selling off its surplus bikes dirt cheap -- right at a time when a whole lot of men came home armed with experiences of riding and boredom to fill. Thus, many big motorcycle clubs, like the Pissed Off Bastards of Bloomington, consisted of veterans that found post-war comfort in the idea of accepting a rank and putting on a uniform. A group of Bastards broke off, and began calling themselves the "Hells Angels" after a nickname for Air Force squadrons. It makes more sense when describing fighter jets flying through the air, but whatever, you do you.
The early Hells Angels got into mild trouble in the form of bar fights but had nothing to do with organized crime. The transformation came when non-vet Sonny Barger formed the Oakland chapter and realized that if he had a whole lot of men at his disposal, he should probably get them to do something productive, like running guns and cocaine. Barger set the rules for the whole organization, and other motorcycle gangs used the Hells Angels as a model. An so the stage was set for them to turn to pimping, murder, plotting to assassinate Mick Jagger, etc.