It’s a complex issue with a lot of nuance.
Merondill isn’t ancient by any means, he remembers a few hundred years of prosperity and peace with the Humans before the Wars and Dark Portal; he remembers how fondly he looks back at those times of utter peace and contentment nowadays and perhaps largely due to his faith and idealistic outlook in contrast to the snivelling cynicism of some older thousand-year-old elves he does think there is some sort of aspiration of peace that can be achieved, especially now that Turalyon has managed to secure three years thus far as of Dragonflight of true peace on Azeroth with the Horde - even working together with them in co-operative effort on the Dragon Isles.
But equally, until recently Merondill was Sin’dorei, albeit a priest who largely just spent his time isolated in Quel’Thalas and Silvermoon healing the sick, helping the needy and not involving himself overly with the Horde or Alliance conflicts because they never reached Quel’Thalas itself. What largely drove him over the edge was Teldrassil, then Lordaeron, then Brennadam and, finally, Sylvanas’ sheer insanity: a premeditated genocide of the universe itself, though he is unaware of just how deep the Shadowlands rabbit hole goes, knowing only that what she was up to was evidently no-bueno if it had restarted the Scourge on Azeroth and “shattered the Heavens asunder” to such a degree that the Lich King of all people was part of the efforts to stop her.
As for the individual races of the Horde itself, he has sincerely mixed feelings about a great deal of them:
Orcs are hated outright, the few always having to wrangle the sheer bloodthirst of the vast majority is always incredibly concerning.
Trolls are an ancestral enemy for the most part, though he acknowledges Vol’Jin attempted to be a good leader of a peaceful time and was taken perhaps far too soon and he is thankful for the Darkspear’s shared aid in helping dispatch the Amani.
Tauren he has no great qualms with, even respecting them for their similarly optimistic outlook towards peaceful endeavour and their reverence for nature.
Forsaken? Some of them were likely friends and allies before their undeath, he acknowledges their aid in the Ghostlands and Tranquillien but it contradicts his faith in the Holy Light, now even more so with the return of Calia Menethil; if nothing else, perhaps a cure could one day be found for the cursed denizens of Lordaeron to return them to true life.
The Sin’dorei are wayward kin, but shedding even more Elven blood, especially now, is a grave and unnecessary act he feels; better to wait with hope in his heart that families can one day be reunited in Quel’Thalas again when centuries have passed and the Horde and Alliance are but ancient memories in forgotten tomes.
Goblins are selfish and not to be trusted under any circumstance, they naturally deviate towards defiling nature and causing unnecessary destruction.
Dracthyr who sided with the Horde? It remains to be seen.
