Incase you've forgotten, it was the Alliance that drove Kael to his rebellion against it. After the Third War, the Scourge still roamed and it was Lord Garithos' job to destroy them. With this, he employed the remaining high elves. His racism and hatred towards them forced Kael'thas to seek different sources of protection which is where he found Lady Vashj. If you look back, the high/blood elves have a certain connection with the Naga, like the Naga have a connection with the satyrs and the Kal'dorei. Anyway... because Kael'thas was thinking for the best of his people, Garithos ordered them to be imprisoned and executed. I do not believe the Silvermoon Remnant (of whom remained in Dalaran and eventually were slaughtered, some now remain in Theramore) knew of what took place. Only that Kael'thas rebelled against his allies for the better of his people. Really, it was an act of protecting his people. But now, he plans for revenge. But who wouldn't? In my opinion, it is how Kael'thas expresses his patriotic side.
And the Forsaken... well, they have a reason for vengeance. But they're vendetta is against the Scourge, not the Alliance. That is why they still remain in Lordaeron and not in Kalimdor like the true Horde.