As I said, an important factor is that Blizzard openly declared that it was the Horde (and only the Horde) who killed Zul’jin. They said this probably to make it clear to everyone that the Amani can’t join the Horde anymore.
Even a race like the Blood Elves who was invaded by the Old Horde, but the difference is that the Orcs in Quel’thalas didn’t kill the King or any important character of the Elves like Sylvanas or Lor’themar, they only burned down the forests but later the Ghostlands were ruined by the Scourge and their plague anyway so the Blood Elves decided they could join regardless of what happened in the Second War.
It’s not just the Blood Elves. The Amani hate almost every race in the Horde.
They hate the Orcs because they feel abandoned in the Second War when they were just going to defeat the High Elves. They probably hate them regardless if they are the green ones or Mag’har as it would be difficult for them to understand the difference, probably.
They hate the Darkspears because they are Jungle Trolls and because apparently there was a long conflict between them.
They hate the Zandalari because the Zandalari chose the Horde over them eventually.
They hate the Revantusk because they joined the Horde and are now seen as traitors.
They hate the Forsaken because they are former Humans of Lordaeron who fought against them in the Second War, and because they are undead now who still live in Lordaeron (Lordaeron was part of the Amani Empire too), and because until recently their leader was Sylvanas who hated Forest Trolls.
They hate the Blood Elves and the Void Elves too considering they were exiled in the Ghostlands closer to Zul’Aman.
They hate the Ogres and the Goblins because just like the Orcs, they feel abandoned by them in the Second War (even if it was the Steamwheedle and not Bilgewater back then, but they still hate the race as a whole).
They hate the Nightborne because they are Elves who grew so close to the Blood Elves recently, helping them a lot.
and probably more. The only ones that the Amani don’t hate are probably the Tauren, the Pandaren and the Vulpera, and that’s just because they lived on another continent and never were in contact with them directly, otherwise they would hate them too…and if I’m not mistaken one of the quotes of Zul’jin about the Blood Elves joining the Horde actually is:
“I hate you. I hate you ALL…”