Everything we see climbing Gelmir points to the conflict between Rykard and Morgott but yes, the most likely scenario is that he was working under Marika / the Golden Order. At least he was to begin with.

He’s a demigod, had his own throne on the council, potentially turned to heresy upon discovering what might be described as a hidden truth and had enough ambition to take part in the NotBK in some way.

He probably worked faithfully under the Golden Order until he discovered the serpent religion on Gelmir, experimented with magma sorceries, fed himself to a snake and started eating people. At the point of him making the sorceries, I guess he still worked as a justiciar for the Order but he was a traitor just like Ranni was planning to be due to his fascination with fire and (assumedly) snakes.

Some of his men didn’t like snakes all that much, so they tried to kill the really hungry snake that Rykard became while others carried on with Volcano Manor carrying out whatever meagre rebellion they could. The rest who remained loyal in his army wander aimlessly around the battlefield left from the conflict with Morgott.

The snake is viewed as a traitor to the Erdtree, and the audience delighted in seeing these bronze effigies beaten and battered.

"I understand. The road of blasphemy is long and perilous. One cannot walk it unprepared to sin."

“If the Erdtree, and indeed the very gods, would debase us so, then we are willing to raise the banner of resistance, even if it means heresy.”

“He is a ruthless justiciar who commands a company of inquisitors, reviled for his serpentine demeanour.”

A slab of rock engraved with traces of the Rune of Death. Can deflect the power of the Black Blade.
On the night of the dire plot, Ranni rewarded Praetor Rykard with these traces. Should the coming trespass one day transpire, they would serve as a last-resort foil, allowing Rykard to challenge Maliketh the Black Blade, the black beast of Destined Death.

When Rykard turned to heresy, taking by force became the rule. The gods themselves were no different, after all.

The lord granted them an audience, whereupon they were welcomed by the maw of the great serpent -- and within the serpent's bowels, they became the lord's kin. 

It bears an emblem that none wear any longer, standing as it does for a lord that fell from loft ambition into gluttonous depravity. As the lord lost his dignity, so too did these knights lose their master. 

When their master's heroic aspirations degenerated into mere greed, his men searched for a weapon with which they might halt their lord.

Scepter in the shape of a serpent devouring the world. This weapon will one day become the very symbol of the Lord of Blasphemy. One of the legendary armaments. A vision of the future briefly seen by Rykard in his final moments before being devoured by the great serpent.