In Dark of the Moon, we see that Sentinel Prime is good at the first half of the film. But at the near climax, Sentinel Prime betrays Ironhide. As we see in the movie, Sentinel Prime is an Autobot - and yet he agreed with Megatron to resurrect Cybertron.
Why did he change his allegiance?
Why?
It looks like that the Autobots consider Earth their new home, and they don't want to rebuild Cybertron. Decepticons, instead, want to rebuild Cybertron. Sentinel's love for his home planet overcomes his loyalty to his fellows Autobots. He explains it in the movie (even if I acknowledge that dialogues often get lost between Bay's beloved explosions):
Optimus: Why, Sentinel, why?
Sentinel: For Cybertron! For our home! What war destroyed, we can rebuild! But only if we join with the Decepticons.
Optimus: No, it’s not the only way. This is our home! We must defend the humans!
Sentinel: So lost you are, Optimus. On Cybertron, we were gods. And here, they call us machines. Let the humans serve us, or perish.
Autobots consider humans their new friends, which apparently Sentinel can't accept, because he considers them weak. So he probably considers weak also the Autobots, and so he allies with the one he considers stronger: Megatron.
I don't remember if it is explained in the movie (you know, "BOOM > dialogue") 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 from the dying Cybertron. So he decided to double-cross his allies even before setting foot on Earth, what happened on Earth later (Autobots friends with humans) only made him angrier and his betrayal more vehement.
p.s. Sentinel is not the only one that changed side in the Transformers franchise, so his betrayal is not so "strange".
In short... because Ironhide was in his way, and with Optimus not there there was no one there with the firepower to stop him.
In long:
Sentinel betrayed the Autobot cause a very very long time ago on Cybertron. He was a traitor already back then.
The thing I don't understand is why Starscream destroyed the arc with him, but that could have been a double cross there.
Still though Sentinel with what he said his motives were sounded more like a Decepticon than an Autobot (he wanted to be a god again.....and he was only eager to share so far as he had to).
With Optimus not at the base, and Sentinel needing the contained artifacts from there for his plan to work, he only had to convert or get rid of the Autobots there. And as Autobots usually are loyal... it was clear that it would be better to eliminate them in a surprise strike. So he killed Ironhide (according to what I read he even killed the "twins" from Transformers 2). And also anyone who stood in his way.
So all in all... no surprise betrayal if someone knows the truth, but more sort of an ambush by a camouflaged (as Autobot) enemy. And Ironhide just stood in the way.
Sentinel prime was never good, he was working with the Deceptions the whole time.
