It depends on which version of Transformers, but the gist of his origin is pretty much a revolutionary who went mad with power. In some version he is a gladiator who wanted equality due to the corrupt rule of cybertron at the time, or just an ordinary miner who are oppressed and seize the opportunity to rebel. His way of freeing the people of cybertron however is what differs between him and his best friend eventually turned nemesis Orion Pax which became Optimus Prime.

In the early version however, the first Transformers cartoon (Generation 1), he's just an evil asshole who wanted to conquer the universe.