Official Canon Edit
When Angron arrived on his homeworld, he suffered severe head trauma; those who found him implanted devices known as "Butcher's Nails" into his brain and made him into a gladiatorial slave to fight for their amusement. The nails delivered constant pain unless he was neck-deep in violence and gore. Desperate to free himself and his fellow gladiators, Angron lead a slave rebellion that threw the world into chaos, until his army was finally beaten back and was about to be slaughtered.
It was at this time that the Emperor found Angron. Angron refused to go with his father, as he preferred to die with his men than watch them get slaughtered. The Emperor ignored his wishes and teleported him off planet. The Emperor could do nothing about his nails, as removing them would lead to a very messy death. The Emperor tried to give command of the World Eaters to Angron, but Angron killed every captain that met with him until cool-headed Kharn convinced him to lead them. Angron would be used to punish non-compliant worlds, as the savagery of his legion would leave none alive.
Horus Heresy Edit
Angron sided with Horus from the start, still furious with his father. Angron would rescue his brother Lorgar by knocking over a Warhound Titan that attempted to step on him. Lorgar would return the favor by transforming Angron into a Daemon Prince of Khorne.
Later History Edit
Angron personally lead the First War of Armageddon, a massive land battle where he and a large combined force of Daemons and Chaos Space Marines attempted to destroy the hive world Armageddon. It took massive numbers of Imperial forces to finally banish him back into the Warp.
TTS Canon Edit
The Emperor stated that "Angron was constantly mad. Like, extremely, unreasonably, earth-shatteringly fuck-ass mad. Probably from all those nails in his brain."