They weren't true Sylvanas fans, only the bandwagon fanbois that both sides hate. It's like football, true fans will stick with their team regardless of if they win the Super Bowl or if they just had another shitty year (I'm looking at you, Raiders! Get your act together already and win your fans another Super Bowl!) Same thing with Sylvanas. I personally don't think she's evil still, but I would consider her an anti-hero similar to Illidan. For them, the end justifies the means to get there.
It's like Arthas in Stratholme. He knew that the Scourge had plagued the grains and that the citizens already infected inside the city were certain to succumb to a fate worse than death. Uther and others pleaded that instead they should wait it out and give them the chance to overcome it. Arthas took the risk to help give un-infested citizens a chance at survival by killing those already plagued because if the plagued did succumb it would mean certain death for everyone in the walls instead of just them.
You also have a similar scenario in Starcraft 2: Sons of Liberty where you have to decide if the citizens that escaped the zerg and created a colony on Haven should be given a chance to be cured or purged. The protoss said that they found corruption on them and wanted to eradicate them all to prevent the infestation from spreading, while Dr. Hansen pleaded with you to give her some more time to find a cure and that she was certain to be close to it.
If we hold the following true that Sylvanas' endgame is to stop the Old Gods from taking over all of Azeroth, and that she knows from some source (which we still do not know for certain who or what told her) that only death and the Shadowlands can be the dagger to stop them once and for all, then the following must be asked: If you were in her scenario, what would you do to ensure that happens? What would you be willing to do in order to 1) bring both factions together, and 2) stop the Old Gods? Both factions have been at war since the Orcs first invaded Azeroth and while there have been some truces/peacefires, there has never been a unification or complete willingness to work together between the two factions. Even in the Ahn'Qiraj war, they put their differences aside only long enough to stop a bigger threat to both of them. They quickly went back to fighting each other though.
I'm not saying Sylvanas is benevolent or did this out of some altruistic reason, and honestly I might be giving Blizzard writers too much benefit of the doubt consider how much they've fucked up stories beforehand that make GoT's D&D writing duo look like geniuses in their last season. But until Blizzard fucks up Sylvanas' story and creates her into a generic villain, I'm giving our Dark Lady faith that she is doing what is needed to be done to bring both factions together and stop the Void from devouring our world. After the events on Argus and with the imprisoning of Sargeras, we no longer have our world shrouded from the vast emptiness that fills our universe anymore. The void will come, it's a matter of when not if at this point. You don't have to agree with Sylvanas becoming a Dark Knight to unite the "Gotham" of Azeroth together but to say that the only possible scenario that an expert tactician like her would do such things is only because "lawl I'm evil!" is a bit silly.