It was never proven that Briant killed his father; if he was behind it, no proof exists. Certainly, there were plenty who had cause to see him dead. However it happened, Briant became king in 1149.
He was already middle aged at 40 years of age when he made the trek from Angouleme to Castle Rennes, still under repair from the war against Scotland. He was still, in most ways, the same bookish, shy, humble boy his father and mother had alternately ignored and despised, though he had developed a penchant for angry outbursts of cruelty, no doubt from some internalization of the rude criticisms of his family.
He brought with him his wife, Judith, a fat paranoiac who compounded his misery by demanding constant isolation, lest she and her husband be murdered in their beds. Oftentimes, the new queen would sleep alone, barricaded in her room. The two had still not produced an heir upon their arrival at Rennes.
Briant inherited a bad situation. War in Wales was in full swing, a conflict which was started precisely because his father felt he needed to be replaced. He was honor-bound to see it to its conclusion, no matter the initial cause. As well, revolt was once again threatening to break out across Ireland; rumors of his involvement in his father's death had stirred the lords of Ireland against the crown once again.
The first year of his rule saw Connacht revolt, followed by Tyrone in 1150. Briant let them, as all his troops were running roughshod over the Welsh countryside. Wales was the priority, just as it would have been for his father. It had to be seen through to the bloody end.
Perhaps mercifully, King Llewelyn I of Wales died in battle, leaving his his brother on the throne. This more or less eliminated Elinor's claim to the throne and, with it, any questions about Briant's half-brother's uniting of the two kingdoms. Wales sued for peace quickly after Llewelyn's demise, ceding Devon and Cornwall back to Brittany while not granting Elinor her place on the throne. Briant was happy to take this offer.
The king's military attentions quickly turned to Ireland, that bane of Brittany's peace for three generations. The forces of Tyrone and Connacht had grown fat and complacent in the face of meager Breton resistance. This came to a crashing halt in 1152, as first Tyrone and then Connacht are put down on the field of battle. Special ire was reserved for the forces of Connacht, as Duke Gilla-Patraic was executed during one of those rare but dangerous outbursts on the part of the king.
All settled into normality for a time. Briant and Judith finally had a child, a son, named Barthelemi, in 1151. Born as the Irish rebellions were waning, he was quickly betrothed to the 9 year old Duchess Heloise of Burgundy, in the hopes of folding that storied duchy to the kingdom in due time.
In 1155, Connacht rebelled once again, this time led by the deceased Gilla-Patraic's daughter. Her forces were quickly routed aside by the Bretons, but the army of Brittany was called back to the continent by a large peasant rebellion in Penthievere. The peasants were led by a group of flagellants and, after conducting an impromptu pogrom against the county's Jewish population, let themselves be swept up into conducting a siege of the county seat in their fervor.
Briant landed with his forces and fought the peasant rabble at the Battle of St. Brieuc in the summer of 1155. His horse threw him after it was gored with a spear. The dismounted king was set upon by the peasants, who beat him to death with clubs and rocks. He was 46 years old at the time of his death; his son, the new king, was only 4.