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How did the Hell's Angels motorcycle gang get started? I know the guy who ran it was Sonny Barger,I don't know how he started the gang though. And why were they hired for security at the Altamont Rock Festival when it was KNOWN that they were a VIOLENT bunch? And why were ALL cyclists,especially Harley-Davidson owners,pegged as bad people?
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I don't remember exactly when the first Hell's Angels club started.  I think it was in the early '60s.  The "biker" gang as we know it today evolved after World War II.  Prior to the war, "motorcycle clubs" were gentlemanly hobbyist affairs.  After the war, as I've heard it told, you had thousands of grounded and restless "flyboys" who just couldn't resign themselves to a quiet civilian life.  They took to riding instead of flying.
The freedom of the American open road was a lot more satisfying than a 9-5 grind.  But you gotta make money, right?  Well, as bikers grew into a shunned subculture, they also turned to unsavory ways of making a buck...theft, drugs, prostitution, thuggery, and so forth.
This is NOT to tar all bikers as thugs, which was never the case.  However, in the heydey of biker culture in the '60s and '70s, there was a lot more malfeasance by biker gangs.  The biker lifestyle was a risky and wild side of the counterculture.
To be fair, some of the hysteria about marauding bikers was overblown in the mass media, but as you saw from Altamont, bad things could happen to innocent "citizens." when one met the other.
Today, biker culture is but a simulacrum of its past.  "Outlaw" gangs are isolated and short-lived for the most part.  Most of the guys you see on the road with their Harleys today are "weekend bikers" with straight jobs and suburban homes.  I don't say that as a criticism, it's just the way it is.  
Have you seen the sticker price of a Harley-Davidson these days?
BTW, I've never even ridden a motorcycle.  I don't have an insider's POV.  I'm just going by what I've read on the issue.

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Maxwell Smart pretty well nailed it...
The origin of Hell's Angels is kinda foggy, as I'm assuming that's the way they want it...
I've read and seen some bios on the motorcycle club and it's mentioned that the crews of bombers in WWII in europe called themselves "Hell's Angels" because they flew in as angels on clouds of silk to rain down hell (bombs), or that they flew into hell (war/combat).
It was also a used as a term to describe the ultimate gung ho, balls out attitude of some of the fighting men who would do anything to win their fight or battle.
As Max said, the crews from the bombers were looking for the excitement and to avoid the bordom of civilian life and took up the next closed thing...also note that Harley was a major supplier for motorcycles for Army messengers/gophers on the battlefront and was much loved by their Army men that used them, it saved their life or the lifes of others. The same bikes were sold very cheaply as army surplus once the war was over, giving many their first cheap transportation in a car starved US (no car production for most of the war).
A member from one of the first motorcycle clubs, the Booze Hounds (Booze Fighters?) split from the group to form Hell's Angels in the late 40's.
The partly fictionalized and overhyped incident at Hollister, got a negative comment that they (HA's) ruined it for the other law abiding 99% of motorcycle riders. The Hell's Angels took that as a badge of honor, that's what that 1% patch on the jackets represents...
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I saw a History Channel special on the Boozefighters,they were a wild bunch! But,for better or worse,I've always loved stock cars,muscle cars,and cycles,and the AMERICAN made Harley-Davidson will always be my fave motorcycle.