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Sentinel Prime betrays Ironhide by killing him, which was his plan from the beginning since he was a traitor to the Autobots.
Since he killed Ironhide, why not also kill Optimus Prime, the leader of the Autobots?
Later during the battle they fought, but that's another issue because they are in a battle.
Who better to answer this than Sentinel himself?
Sentinel: Commencing transport.
Optimus: Stop! No, no, no Sentinel!
Sentinel: Forgive me.
Optimus: Autobots, retreat. 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. You're lucky I didn't kill you. In time, you'll see.
Optimus: It's not over.
Sentinel gives an implication as to why he spared Optimus. Sentinel was being treated like a "lower-being" on Earth, and being a haughty individual (he was a Prime), didn't like this as well. He wanted to be treated like a God. He wanted Optimus to watch the Cybertronians take over Earth, and subsequently join them in ruling Earth, hence he let Optimus live.
It was because he wanted Optimus to see what he saw and join him. In his mindset Ironhide was in his way and as most others just fleas. But Optimus was different to him. He was Optimus mentor for some time even (as Optimus mentions once) and Optimus even would have given him the Matrix just before the betrayal.
Still though he did NOT interfere when the Decepticons tried to kill all of the Autobots including Optimus when they blew up the ship.
So all in all: he had no specific desire to kill Optimus, nor did Optimus directly stand in his way. Else he would have wasted no microsecond in an attempt to kill him (like he did in the final battle).
Sentinel Prime never wasted a single micro second to kill Optimus, thus Sentinel was the predecessor of Optimus, and Optimus offered him the Matrix of Leadership, but Sentinel refused to accept it.
He betrayed the Autobots due to bringing back his very own planet, Cybertron. When Sentinel's ark was crash-landed on the moon, Soundwave and his evil Decepticons took all the pillars except 5, including the control pillar.
The only way to revive him needed the matrix. This is the thing Optimus affected to Sentinel and revived him, and he knows that the Deceptions had all other the pillars so he betrayed.
Ironhide is the only badass Autobot, which is the strongest one and the most feared bot, so Sentinel blasted him with his sonic rust cannon.
Sentinel needed to be treated like a god; in the final battle he says to Optimus "we were gods once all of us (the 13 primes), but here there will be one". Optimus tried to stop transporting the resources so it made Sentinel a little bit angry.
