The farmers from Northern California who settled here a long time ago (probably before the 1950's) were patient and slow people and took everything with ease. When they communicated to explain "VERY" they would say "hell of a" instead of "very" or "a lot". Then the city folks who migrated in later to the Bay Area took the Northern California farmers saying and shortened it to "Hella" instead of "Hell of a."
So in the end the word "hella" comes from the Bay Area city folks. Short and sweet.
She/he was hella sweet.
It was hella cool.
The ride was hella fresh.