Some people say the HA won't bother you if you don't bother them - but that is a generalisation, and not always true in practice.
The original HA's were formed from a bunch of ex-B17 bombers after WWII, who rode together 'cos they wanted their wartime camaraderie to go on. The organisation grew. It is alleged that the organisation made lots of money out of drugs. The police claimed (in May 1993) that they were the fastest growing criminal organisation in the UK. The HA deny this and say they're being scapegoated 'cos they're obviously easy targets.
Hells Angel and the HA design are a registered trademarks so legally any non-HA would be in trouble wearing 'em. In practice any non-HA would be in DEEP shit, at least in the UK. So don't go and tip-ex "Angel from Hell" or something like that on the back of your leather jacket, like the much-behaired Angelos was considering. (Angelos is Greek and over there the conventions are different - apparently any hairy biker can put on in HA-like stuff and not get hassled by the HA there).