Yeah, but Daelin Proudmoore’s character isn’t solely from what was shown in Warcraft III. That’s like saying Jaina is only her character from Warcraft III, or that Thrall is only his character as shown in Warcraft III. The character’s being includes all material they’ve been shown in.
Obviously, he’s an antagonist to the Horde, especially in Warcraft III. That was never in question, nor should it be. But your freedom fighters are my terrorists, and vice versa. Daelin Proudmoore, with the context given from all other materials, is a tragic villain, because we have other content that shows how he was outside of his feud with the Horde. Also, had the Horde he waged war with been any Horde other than Thrall’s Horde, he may very well have been right in his actions. What made him wrong in this scenario was how Thrall’s Horde sought redemption for what they did while under the Legion’s control, had turned away from the old Horde’s ways and in fact stood side-by-side with Jaina’s Alliance expedition against the Burning Legion to save the world, yet Daelin wouldn’t hear any of it because of his own flaws.
For an adequate comparison, look at Arthas and Garithos, two other Alliance-originating antagonists. Arthas was a tragic antagonist. Garithos was not.
Arthas was driven by his desire to save his people, avenge the fallen, protect his land at all costs. He loved his family, he loved his father, he loved Jaina, he loved his people and even as a kid played with the common folk and talked with the guards. Outside of his campaign to save Lordaeron, he was shown to be a good man. Then the Culling happened. Let’s be honest here, it was the only real option. But it still resulted in countless civilian deaths. And up until he picked up Frostmourne, one could say that it was still well-intentioned, if very extreme solution that could’ve potentially succeeded. Because it was all to save Lordaeron. He was doomed by his own feelings, by his own desire for vengeance and his own patriotism. Even after picking up the sword, there’s still a tragedy to it since he became his own antithesis and committed actions that went against everything he stood for even seconds before picking up Frostmourne, all against his will with his soul stored in the runeblade. He was a tragic villain.
Garithos was driven by racism. The most redeemable thing about him comes from one of his hero quotes, “In memory of Terenas!”, which implies a lasting loyalty to Lordaeron, the Menethil line and King Terenas in particular. Out of game material explains that the reason for his extreme racism towards elves in particular is because he felt they diverted Alliance resources when his home, family and people were destroyed while he was stationed in Quel’Thalas, which is something that may very well have happened anyways had he been there. Nothing points to him having been a pleasant person before death, and no one will ever look back and see him as a heroic figure of any sort. He was not a tragic villain.
Because of the lore behind Warcraft II and Warcraft III, one can see Daelin as a heroic figure during the battle with the old Horde, and Arthas as a heroic figure during the battle with the remnants of the old Horde and later on the Scourge. Even their feelings as they commit the atrocities they did can be relatable to many, because it doesn’t stem merely from “well they’re different so I hate them”, but from their own past interactions with the elements they’re fighting that shaped their views on the matter. That makes it tragic when they become antagonists precisely because of those feelings.
In contrast, with the lore behind Garithos there is nothing remotely heroic about him other than being the leader of the Alliance forces against the Scourge in Lordaeron, which could be considered a noble goal at least. But that doesn’t equate to the man himself. He had no reason to hate Kael’thas other than him not being human. There’s no tragic depth to how he became a racist other than him blaming the elves for his home burning down.
But despite that, Daelin’s actions are never painted as being justified and right at the time he committed them. The dialogue chosen to be remade in BFA for the Drust visions are all filled with his extreme anger, Jaina trying to convince her father not to do it, and lamenting about him not letting go while cradling his dead body - all lines that originate from Warcraft III that, in context, show him as having been wrong about Thrall’s Horde and how his hatred got him killed.
Chances are, had Daelin attempted the same thing against Garrosh’s Horde or Sylvanas’ Horde, he’d be painted as being more of a hero standing up against them than a villain. But against Thrall’s Horde, he has been and always will be the antagonist by default. But that doesn’t make him any less tragic, story-wise. And if the new Warcraft III changes that, then they’re missing the point.