The following comment consists of a lot of assumptions, guesses and conclusions from me about what we learned from Merus in this chapter.
Let us see:
Merus stated that the gods do not mind a “bit of chaos” as long as the world’s balance is retained.
That “bit of chaos” includes world domination, discrimination against other races and even exterminating one or more races altogether.
Then he stated that the gods sent a “negotiator” – the god of conciliation – to the demons.
With the first statement about what is acceptable for the gods = the “bit of chaos” means that the gods would have no issue to make the offer “As long as you (the demon king) guarantee that the world’s balance is kept, we (the gods) will not interfere in your actions even if they include world domination, discrimination against non-demons and the extermination of the human race or/and other races.”.
Even if the demon king intended to destroy the world’s balance eventually, he would most likely accept that offer to gain time to erase all opposition in the world itself, securing his rulership over the world and strengthening his forces before eventually breaking the world’s balance or attacking the gods.
But the demon king refused the “offer” of the gods.
That implies that the gods most likely gave a condition about sealing a part of the demon king’s powers or putting a restrictive magic seal on him or so.
That would be the only explanation why the demon king would not accept such an “offer”.
With the demon forces already being able to keep the other races in check without the high ranking demon generals, s-rank monsters or the demon king himself, the gods showing the intention of weakening the demon king are the higher threat.
As such that also explains why he then organized an attack on the gods’ realm.
Lastly if the gods are concerned about the power of the demon king being able to destroy the world’s balance, that means whoever defeats the demon king and his forces would be an even greater potential threat to that balance and therefore even of even higher concern for the gods.
So Merus – the angel of the highest ranking god – becoming a summon of Allen who plans on defeating the demon king is actually most likely also meant to be a watchdog over Allen.
When Allen then has his final fight against the demon king, Merus could then possibly report the result of that battle to the gods directly.
No matter who wins the battle they would be weakened and the gods would then have an easy chance to get rid of the winner.
Maybe once the final battle against the demon king is over, Merus could unleash his orginal power to attack and kill the weakened survivor – no matter if it is Allen or the demon king.
That is my assumption about the plan of the gods and why Merus actually became Allen’s summon.