Okay OP, since you asked an honest question, I’m going to lay it out for you in as unbiased a way as possible, which is far and away too hard for many people on this forum. This will be long so bear with me.
The Purge of Dalaran is essentially the only actually morally grey moment in WoW’s history, and whether by accident or on purpose, the writers thusly made it quite complex in terms of various factors. The backdrop is what can essentially be called a war campaign for both the Horde and the Alliance, wherein they progressed through a chain of quests dealing with the other faction in Pandaria and abroad, centered around a powerful mogu artifact called the Divine Bell.
The Divine Bell essentially has the ability to create supersoldiers when rung, and seemingly had no limit to the number of beings it could empower. At this time in the game, Garrosh had wiped Theramore off the map using a mana bomb, a weapon of horrific destructive potential, using the help of a Sunreaver, or Sunreavers, to make it. Dalaran had already been present at Theramore, because they had learned the Focusing Iris was taken, and it’s their express purpose to try and make sure the factions don’t use big scary world ending magic. In fact, it was Aethas himself who was the tiebreaker on the council vote as to whether or not to go help Theramore and evac people if need be.
The quest campaign takes place roughly six months after the destruction of Theramore, and Dalaran has been basically watching for months and not really intervening at all, despite Jaina being the one in charge. At this point, it’s become clear that of the two factions, the Horde under Garrosh was far more likely to use the Divine Bell, and annihilation was on the table if Garrosh was to get it.
Now, to the Purge itself, the general concept is this. Dalaran had given the Divine Bell when found to the Alliance, because it needed a certain thing to activate it, that the Horde had. One side had the way to activate it, the other side had the item itself, neutralizing it as a threat. Using Dalaran’s portal network, a Sunreaver and Horde agents stole the Bell back, and thus gave it to Garrosh.
Here’s the thing, people will argue about neutrality this and neutrality that, but really? Dalaran’s neutrality is not the issue at hand.
Garrosh had been shown to already absolutely be willing to wipe out cities using powerful magic artifacts. This is not in question. Jaina had absolute knowledge that it was a Sunreaver who helped, and no idea how many agents were in Dalaran willing to also help. We operate on a meta knowledge level, but as far as anyone knew, total destruction was on the table. Thus the actual Purge was locking down the city of any blood elf, because there simply wasn’t time to waste, or to do it in a measured way.
This is why the Purge is the only morally grey event in WoW, because there was no correct action to take. To wait and see would have possibly meant everyone on Dalaran being wiped out, but the Purge itself was very much not a good thing either. The characters involved were essentially backed into a corner on all fronts, and thus made a decision that wasn’t justified in hindsight.
People often miss a few things because there’s that inherent bias for their faction, as well as a very odd perennial dislike of Jaina that pre-dates MoP and started in Wrath. One, the only reason the Purge didn’t become totally justified later on was because Anduin intervened and destroyed the Bell in a sacrificial Hail Mary. Without him doing that, Garrosh explicitly would have won the war and more than likely thousands, if not tens of thousands would have been wiped out. Two, both the Mana Bomb and the theft of the Divine Bell happened under Lor’themar’s watch, and yet he seems to never get flack for it. Love him as a character, but I’ve found this strange, because he completely drops the ball twice and nearly causes catastrophe, but he just flies under the radar.
As for the effects? Many on this forum with overstate, or understate them, but the truth is we really have no indication it was as catastrophic or horrible as many claim. Unlike some, I’m fully convinced that some blood elves did die, there’s no question there, but the numbers are difficult to surmise, as the Sunreavers and all the named blood elf NPCs are immediately wanting to come back, without question. All the blood elf NPCs in Wrath Dalaran are the same as Legion Dalaran, which makes canon deaths difficult to figure, and also puts a damper on how bad it might have been since they don’t seem to hold any grudges at all. We do have, in BfA, one single instance of some Sunreavers attacking Jaina and calling her a murderer, but that seems to be more a reaching attempt by the writers to try and at some ‘greyness’ to everything. “Hey guys we know we’re making the Horde look really villainous this xpac, but uh… remember when Jaina did something bad???” Lor’themar also works with Jaina the most readily out of all of the Horde leaders, and given how few blood elves are left, one would think that he’d be far more vengeful of any marked number had died.
Also immediately after being let in another group of blood elves betrayed Dalaran to work for the Legion but we don’t talk about that…
TL;DR: WoW players want morally grey and nuanced writing but can’t actually handle it when they get it.