Yhorm was asked to lead the nation his ancestors conquered by the people that were conquered. He had a strong sense of duty and was a very forthright being. When he lost the person most important to him that he wanted to protect, he abandoned the use of his shield. When he wanted humans who doubted him to trust him, he gave them a weapon that was his greatest weakness, the Storm Ruler.
Yhorm likely chose to link the flame because of his sense of duty and loyalty to the people he ruled. That's why he abandoned his duty upon resurrection; when he linked the flame, everything went wrong. He believed he had made a terrible mistake.
The bonfires were intrinsically linked to humanity by Gwyn, and humanity is the fuel that keeps the fire going. But, bad things happen when you use the wrong fuel, like that of a Giant. The Witch of Izalith attempted to recreate the first flame and instead gave birth to the Bed of Chaos, turning the city of Izalith into hell and bringing demons into the world. Gwyn attempted to link the fire himself, but all it accomplished was to consume his power and turn him into a living husk.
So what happened when Yhorm, a giant, attempted to link the fire? In the room where Yhorm has retired to, you can see scorched human corpses piled up everywhere. The Profaned Capital was consumed by fire after Yhorm the Giant became a Lord of Cinder. The fire, born of the sky, is said to have incinerated naught but human flesh. Though inadvertently, Yhorm regretfully destroyed his country and killed his people. He was betrayed by the fire, and he blames himself.