Garithos sent the Blood Elves to retake the observatories in Dalaran, and defend them. After they worked with the Naga, he told them dont do it again. They worked with the Naga again, aka treason. He may have been a dick, but the Blood Elves sealed their own fate when they committed treason.
Orcs were prisoners, not slaves.
Stormwind screwing over the Defias was because of Onyxia. Varian even tried to help them, but the nobles under Onyxia's influence over rid him.