One of the less reputable aspects of Cybertron before the Great War was the fact that it had gladiatorial combat. Though many of these games were officially illegal, they often thrived, with Transformers battling each other to the death. Many government officials knew of this, but they often turned a blind eye to the bloodsport.
They should have paid better attention, for within these arenas, Megatron first arose... and found many similarly-minded individuals.
Generation One UK Comics continuity
The Gladiatorial Games of Cybertron were ancient athletic contests meant to demonstrate the combatants' skill and ability. Gladiators usually wore battle masks, and fought with shields and hand-to-hand energon weapons.
During the reign of the final Overlord of Cybertron, the planet was experiencing much political strife. Overpopulation had led to a fuel shortage. The larger cities of Cybertron had become sovereign city-states, and the Overlords had become little more than figureheads. In an attempt to garner some good will between city-states, the last of the Overlords had revived the games, starting an inter-city competition. The attempt backfired and it seemed to excite rivalries, especially between Iacon and Tarn.
During the last games of this era, a Tarn gladiator named Megatron brutalized his Iacon opponent Sunstreaker, threatening to kill him. Optimus Prime entered the arena to put a stop to this. Despite an axe to the chest, Prime managed to get Megatron to stop with his photon pistol. This altercation caused the Overlord to postpone the awards ceremony for this games.
At the same time, saboteurs from Vos were using the games as cover for an operation to destroy Tarn's main power plant and frame Iacon for the deed. They were caught by a technician from Tarn who recognized them as members of the Vos gladiatorial team. The power plant was still destroyed, bringing Vos and Tarn to war.
The games did not surface for another 4 million years. The Decepticons revived the gladiatorial combat in an alliance with the alien known as Zabra.
Generation One animated continuity
Millions of years in the past, when Cybertron was still a Quintesson-ruled factory world, the planet's robot slaves were often forced to battle each other in gladiatorial combat for the amusement of their masters. Some of the robots forced to compete in these sadistic games grew tired of it, and attempted to fight back against their masters. Although this initial revolt failed, it would plant the seeds of rebellion that would eventually prove successful, and one of the participants would go on to hold the Matrix of Leadership.
There were several gladiatorial combats staged in city-states far from Iacon, such as in Kaon. For some, such as Grimlock, it was a good outlet for anger and aggression. Megatron was a participant of these games. As his victories mounted, he realized that they were a simple conspiracy by the Council of Ancients to keep the masses distracted. Megatron began using the matches to find similarly-minded Transformers. For over half a million years, Megatron used the games to recruit the Decepticon army, bribing Emirate Xeon to remain quiet. Finally, however, they struck.
After their mine riot and escape, Megatron, Rumble, and Frenzy escaped to Kaon, where they joined Clench's illegal gladiatorial circuit. Megatron's star began to rise, attracting the attention of Sentinel Prime and Senator Ratbat. In Megatron's first victory (against Cy-Kill), the crowd chanted "'Til all are one". They like saying that when people die. As Megatron's power grew, he used the games to recruit his army of Decepticons.