I'm looking at some of the Forsaken-lore from WoWpedia.
"Recently, the Argent Dawn has offered assistance to Sylvanas' people against the Crusaders' constant harassment in eastern Tirisfal Glades."
"The humans and dwarves of the Alliance, the perennial enemy of their Horde allies, remained at large on the continent."
"The Forsaken Defilers seek to foil the AllianceLeague of Arathor in the Battleground Arathi Basin and claim it for the Horde."
"the Forsaken launched an offensive against Gilneas, after being ordered by Garrosh Hellscream to do so. The Forsaken appear to be taking on a more offensive stance regarding the war with the Alliance"
"Forsaken have even less love for the Alliance, particularly because they clash constantly with the human organization called the Scarlet Crusade."
Conclusion: Maybe they don't hate them that much, but it's more because they are Horde now and are 'forced' to be anti-Alliance?
Then why did Sylvanas had to 'bribe' him before he would join them in a fight against the Burning Legion?Also garithos' prejudices did not prevent him from working with forsaken.