As a person who roleplays characters from the Scarlet Crusade, there seems to be a hell of a lot of misconceptions floating around (mainly from Tumblr) and I would like to address a few of them. 

1. They are formed from the Knights of the Silver hand. They are not anti-other races but were formed after the first war, when the Alliance was literally Humans, High Elves and Dwarves which is why they were literally just those.

2. They are not hated or enemies by the Alliance, nor are they allied with it either. They had a common cause.

3. Their Crusade is against the Scourge, Forsaken and any undead. Only. It has no ties to any real world events. They just want to eradicate undead. 

4. They follow the Light vigorously. They were the first order of Priests to actually use the Light to harm the living since the Light could only heal and that’s it (hence why paladins were created because humans lost 90% of their Priests) but the Crusade found a way to distort it through faith to harm. 

5. Scarlet Crusade were almost wiped from existence when the Death Knights rained down upon them. Only a select elite fled to Northrend to continue as the Scarlet Onslaught. Who were evil and enemies to both Alliance and Horde as they kill relentlessly. Scarlet Crusade survivors HATE Scarlet Onslaught.

6. Scarlet Crusade were insane. They were forced to be paranoid about everyone disguising themselves as Undead, since their stronghold was in Forsaken lands and were constantly at war. Majority were honorable but misguided. Balnazzar tried to bend the Crusade to what he wanted.

7. The Scarlet crusade routinely purged their own ranks of weakness. Which is why they have a very elite set of Paladins, Priests, Mages and Monks. They were not against magic. Just undead.