I know this sounds crazy; hopefully just less crazy than the Heralds themselves.
What makes me think this is three things:
The perfect gemstone called Honor's Drop
Reminds me of Atium, Ba Ado Mishram and the Unmade, and even Leras giving some of his power up to humans
Odiums quote "No, we killed you. We killed you!"
The "We" seems really strange. Hard to believe Rayse would share the spotlight with the Fused or even the unmade.
The cause of the Recreance
The motivation can't be as simple as "The Aharietiam was a lie".
The basic idea is this: The Heralds were tired of the war and tortured so they abandoned the Oathpact. When they did so, they wounded Honor. Even while wounded the Oathpack still stood with Tal and Odium was trapped so he could not exploit this.
The Heralds eventually wanted to escape the Oathpact entirely. They live in constant fear that Tal will give in or that one of them will be killed. So they decide, perhaps on the whispers of Odium, that the only way to do to this is to break their Bond to Honor; and thus capture or kill Honor.
Conversely Honor wants force the Heralds to remember their oath. Remember why they fought/fight. Honor conspires with Ba Ado Mishram to give her power while Odium is trapped. He gives her the kind of power of bonds; the power to form connections with the Parsh. This starts the false desolation. Honor plays both sides
During this time the Heralds experiment with trapping Honor's power and leads them to create the Honor's Drop. Odium is crucial for the insights and urgings. Eventually Melishi traps Mishram in a perfect gemstone and then traps her in the spiritual realm. But the power that was trapped was not Mishram's alone. It was in part honor. Between power needed to capture Odium, the broken oaths, the siphoning by the Heralds earlier, the trapping of Mishram was the straw the broke the camels back. Honor was killed; and the Heralds helped do it.
It is the totality of this that broke the Radiants:
Humans were the voidbringers
The heralds destroyed their home and then abandoned the oathpact
They caused the enslavement of a whole people
Honor was the source of Mishram's power
From there the Radiants decided to abandon their oaths. But since Honor had been so weakened and the power of connection so twisted with Mishram, this was the final act that killed him.