So recently when in Stormwind, I noticed two death knights riding through the gates, and the Stormwind guards shouting at them and throwing things at them.. One thing I want to know is, why? From what I've read about DK's, they're a 'hero' class, and as they are available for both factions, I don't see why the guards are shouting at them, saying 'Their family was wiped out by scourge' when they were human and draenei Death Knights..
Could someone please tell me why this is? o.o

Ok so DK;s were the army of the Lich King When Arthas became lich King But before that he was a DK himself and work for the Burning Legion. This was after He was good he got forstmore the sword and became evil and killed his father but this all happens in the warcraft games. So in teh Warcraft games the DKs are killing people and making midless zombies You know that big long dark streak in the Blood elf lands called the dead scar/ that was left over by the scorge so Naturally people would throw things DKs even if you are on the allience side they don't understand that the lich king has no control over you any more. that is why.

The playable Death Knights in World of Warcraft are former agents of the scourge who regained their will when the Lich King betrayed them and sent them to their death. They were spared by the Paladins they were sent to kill and then went back to their respective factions to fight the Lich King.

The guards and townsfolk only yell at them before they officially join the Alliance or Horde by handing in the quest to do so, afterwards they're cool with them.