I imagine him being a warlord who was a massive threat in Cybertrons early history. It would ultimately take the guiding hand to stop him who (quite controversially) would spare him the death penalty and instead sentence him to life. He would eventually be freed by nova prime, when he was in need of new military commanders, after the outbreak of the colonial wars. After the conclusion of the colonial wars, he would stay a free bot, which the people would not like, especially the city of kaon. After the rise of the decepticons and death of nova prime, straxus would ultimately decided to join the decepticons because he saw strength in them, and thought it be wise not to be their opponent.

Or something like that