The actions of the Forsaken in Vanilla World of Warcraft to their gains in Cataclysm cannot be justified, but they can be explained.
The Forsaken concept of their “rejection” isn’t being told you can’t sit at the living kid’s table. It was being treated as no less than beasts and monsters for having the gall to die at the hands of Arthas’ forces, and be forced to commit atrocities in his name through magic. The original Forsaken story posits that none save Thrall and Cairne, are willing to believe they’re not just some sort of Scourge trick at trying to get inside the walls with smiles and speech so they can continue the Lich King’s crusade against all of Azeroth.
You also share your homeland with your living counterparts who are universally hostile and hang you from the trees or torture for sport. A Forsaken goodbye is “Beware the Living” for good reason. You’re not an individual deserving of liberty or (un)life, you’re a threat.
It certainly didn’t help that Sylvanas made the big-brain decision to keep a Dreadlord on as an advisor, given that she believed she had cornered him. She obviously didn’t know about Dreadlords effectively being immortal and unkillable, and Varimathras took advantage of this to wage war on Lordaeron’s living at all costs, even going so far as to have Forsaken defectors hunted and slain when the Alliance would take them prisoner instead of outright killing them. In the end, the Forsaken perpetrated the Scourge crimes upon their living neighbors, which is indefensible.
But when you draw back and look at the larger picture, the Scarlet Crusade, which was and remains the Forsaken’s primary antagonist, was lead by another Dreadlord. They were playing both sides against each other like a game, and this ploy only ended with the sudden intervention from the Argent Dawn when they uncovered Balnazzar in Stratholme and put him down. Sadly by that point, the Scarlets were self-perpetuating, and now have expanded their crusade to include Alliance Races like the Worgen as also being an enemy. But they’re literally the result of a Dreadlord plot to commandeer the remnants of the Paladin Orders of Lordaeron, so we shouldn’t expect anything less than them being an eternal thorn in the side of the Forsaken, Worgen, and heck, even the Elves of Quel’thalas.
When it comes to the living humans of Southshore, basically the Forsaken shrugged and assumed that because initial relations were cold at best, to outright hostile, there was no point in changing their standard operating procedure. Just treat them like Scarlets because they’re going to try to kill you all the same. Ultimately, Occam’s’ Razor serves us well here.
So, the whole reason the Forsaken are as vile as they are is that they were lead by a woman who thought she was smarter than everyone else in the room. Much as I adore her Sylvanas was a terrible politician, but her people didn’t elect her as a politician, she was adored for being a ruthless general—and would later flop at that too—and gave them a goal they could focus on, rather than acknowledging their condition.
Basically instead of getting the stitches they needed for their giant gaping wound, they just continuously injected themselves with shots of adrenaline Sylvanas supplied them to stay conscious and moving forward. The Desolate Council is the right step forward, despite my reservations about some of it’s members. I imagine the future will see a lot more cooperation between the living and undead, and a lot less hostility. After all, the Argents proved it was more than possible.
Any Forsaken player or fan who tries to argue that the Forsaken are innocent, is just trolling you. However, this doesn’t make the Forsaken any more or less evil than anyone else in the game. Even the Sludge Fields was considered a step too far which had to be dealt with through pain of death. Too bad Blizzard forgot the warden was killed and just brought him back with no explanation in a throwaway BFA Wartable mission. Hopefully he died painfully again if that quest is to be considered cannon and not a complete mistake.
I don’t condone what the Forsaken did before Shadowlands, but I won’t apologize for it either because none of us were involved in the quest and story design process.