The answer is Sentinel. He broke humanity’s trust. Optimus Prime spends the entire film on the run not because of his own actions, but because he was guilty by association with his mentor.
Sentinel Prime does not physically appear in Age of Extinction , but his role is expanded in tie-in media:
To understand Age of Extinction , you must revisit the climax of Dark of the Moon . After Sentinel Prime (voiced by Leonard Nimoy) murdered Ironhide and orchestrated the invasion of Chicago, Optimus Prime was forced to execute his mentor. The battle left Sentinel’s body shattered and lifeless in the ruins of the city.
Sentinel's severed head is housed within the laboratory of Kinetic Solutions Incorporated (KSI) .
However, the dark irony is that Galvatron is not better. Because KSI used Sentinel Prime’s "sparkless" metal and overwrote his programming with human code, they left a backdoor. The disembodied brain of Megatron (who survived Dark of the Moon as a head) uploads himself into the Sentinel-derived body. The result is a horrific hybrid: the body of a Prime corrupted by the soul of a Decepticon.
