You guys have a serious problem with Prime being a soldier, don't you? He's the leader of a military that is earth's only (and pretty much last) line of defense against something that they could never take out themselves.
Prime was PISSED. He had seen the war from his planet spill onto an innocent planet's land, granted, the humans in charge of the space exploration lied to the Autobots, but Megatron crashed landed a long, LONG time before NASA existed.
Sentinel also killed one of his best friends, in THE MOST cold blood of actions, point blank with a extremely high powered shotgun, and let him die there...I'd say he was really fed up with the fact that he kept reiterating "I won't stand by and let you take the boy, or his fellow man ______ (insert any decepticon's name"
Also, Prime capturing and basically handcuffing decepticons? Are you effing kidding me? No offense, but either you're seriously hopelessly in love with a flawed justice system and promoting a over idealized, hopeless romantic "cops n robbers" or you're just really put off by doing what NEEDS to be done.
As I recall it was the decepticons who were mercilessly leading the autobots to horrific execution, not the other way around. Prime was tired of having the last of his kind mutilated, maimed and torn apart for some stupid idealogy the decepticons chased after and did what he needed to do.
Haters gonna hate, and Optimus Prime is gonna be Prime.