The Horde should be expunged from Azeroth. They do not fight for the light, and their separation from the Scourge is questionable. They cannot be trusted. Seriously, how long will it be before the Lich King pulls the Forsaken back into the fold?

1) They do fight for the light (here's a kick in the nuts: Blood Elf Paladins are the only ones directly given their power by a Light-based being--otherwise, even evil Scarlet Crusaders and undead under the Lich King's control such as Sir Zeliek can use the Light).
2) The Alliance's separation from the Scourge is questionable. Read: Scarlet Crusade--is the Alliance behind the dozens of ongoing attacks of Crusaders on the Horde? They were also hoodwinked by Onyxia, so Alliance are all under the control of black dragons yes?
3) Killing a whole bunch of innocent people because you think they may end up being your enemy is what turns you into the opposite of "righteous." http://www.wowwiki.com/Arthas_Menethil

@ muffin

"here's a kick in the nuts: Blood Elf Paladins are the only ones directly given their power by a Light-based being"

Did you somewhere along the line completely forget about the Draenei? They've been weilding that power at least since their time on Draenor.

http://www.wowwiki.com/Draenei :
"They followed the path of the Holy Light of Creation, as shown to them by the naaru."

You may be forgiven for making such a lapse since the bloodthirsty and merciless Horde admittedly massacred this peaceful race, including women and childen, all but driving them to extinction. Another reason why a member race of the Alliance would be against peace with such an inherently barbaric faction.

(see arcticle for justification for links between Old and New horde)

If you haven't already read Unbroken, the official short story from Blizz on the massacre of the Draenei, and the rise of the shaman do so now:
http://www.worldofwarcraft.com/info/story/unbroken/index.xml

1. I do recall the Blood Elves sucking their righteous power from one of the naaru. When you undergo the Draenei quests, not only do the Blood Elves admit this, but they enjoy it. The result, this naaru goes evil and becomes a boss in Sunwell.

2. The Alliance was not hoodwinked by Onyxia. Not only did we fight out about it, and stop her, but the moment the king arrived he knew immediately and kicked her ass out the door. At least no great important characters were killed from this. I can't say the same for the Forsaken at the Wrath Gate. I miss Bolvar.

3. There is no shame in Arthas' actions at Stratholme. The people were already infected, they would have become undead at any rate. There was no cure. The English did the same thing regarding the Black Plague. Those actions helped stave off the disease from the rest of the country.