He was fighting for it, then he realized Sylvanas wanted to make it into a terrorist organization, so he betrayed the terrorist Horde not HIS Horde.
In Cata he abandoned it to save the literal world, in Pandaria he still had to heal the damage done to the world(according to ToW), In WoD he rejoined us to battle the Iron Horde, he only unjustifiably abandoned us in Legion.
While not a perfect leader by any means, he had to make these tough decisions 3/4 times to abandon the Horde to save the world.
He tried reason, several times.
In fact, reason is literally the ONLY thing Baine does in this game.
-He tried to reason in BtS, before the war begun, in which Sylvanas responded by forcing Baine to cease his diplomatic relationship with Anduin.
-He reasoned at the Battle for Lordaeron, in which Sylvanas responded by telling him heâs welcome to get himself killed along with saurfang.
-He reasoned with her over the raising of Derek Proudmoore and she responds by being a smart @$$ and mocking the Horde phrase âVictory or Deathâ and saying that âdeath is mine to masterâ.
-He reasoned with her even after he committed treason, she responded by murdering Zelling right in front of all the gathered Horde leaders.
Sheâs not listening to his reason, so he must treason.
A problem which was solved through the death of Xera and Anduin reintroducing him to Alonsos Faol.
I mean, I wouldnât call it betrayal, sheâs literally been tirelessly fighting for her people within the twisting nether for thousands of years.
Itâs more like being incredibly out of the loop and disconnected from her people, now knowing what theyâve been through while she was away fighting with her hubby and glowing space goats for thousands of years.
But yeah, youâre right about her not making the effort to understand her people the way they are now. Iâll give you this one.
What does that mean? Sheâs still a leader though.
The thing is, everyone else you listed arenât selfish.
-Thrall left the Horde to save the planet.
-Saurfang betrayed the Horde to save the Horde.
-Baine betrayed the Horde to save the Horde.
-Turalyon abandoned the Alliance to save Azeroth and likely countless other worlds.
-Alleria abandoned Quelâthalas to save them as well as other worlds.
-Tyrande transformed into the Noght Warrior to avenge those she couldnât save.
Each of these people did something selfish for a non-selfish reason. Sylvanas and Gallywix on the other hand literally only does selfish things for selfish reasons, which is why they lack in the leadership department.