Why is Sentinel Prime betraying the Autobots

Why did Sentinel Prime betray Ironhide?

Why?

For Cybertron!

It looks like the Autobots consider Earth their new home and don't want to rebuild Cybertron. Decepticons want to rebuild Cybertron instead. Sentinel's love for his home planet overcomes his loyalty to his Autobots. He explains it in the film (although I acknowledge that dialogues between Bay's beloved explosions are often lost):

Optimus: Why, Sentinel, why?

Sentinel: For Cybertron! For our home! What war destroys, we can rebuild! But only if we join the Decepticons.

Optimus: No, that's not the only way. This is our home! We have to defend the people!

Sentinel: You are so lost, Optimus. We were gods on Cybertron. And here they call us machines. Let the people serve us or perish.

Autobots consider humans their new friends, which Sentinel apparently cannot accept because he thinks they are weak. So he probably thinks the Autobots are weak too, and teams up with whoever he thinks is stronger: Megatron.

But wait, there's more!

I don't remember if it's explained in the movie (you know, "BOOM> Dialog"), but according to the wiki (take the wiki with a grain of salt as usual!) Sentinel betrayed the Autobots many years ago when he fled before the dying Cybertron. So he decided to cross his allies even before he set foot on Earth. What later happened on Earth (Autobot's friends with humans) only made him angrier and his betrayal more vehement.

ps Sentinel isn't the only one who switched sides in the Transformers franchise, so his betrayal isn't that "weird".

Gallifreyan

I like pretty much everything about this answer (last paragraph most). +1 from me, good job!

Thomas

I only have one squirrel with the answer. This Sentinel did it for Cybertron. In the film he says a few times that he wants to be seen as God. And since Prime doesn't share his vision, he'll be the ONLY God there is. So it's not for Cybertron ... that's just an excuse imho, he's as power hungry as the next Decepticon and was never worth the Autobot icon.

Teem Porary

@ Thomas, maybe I should reconsider my answer ...