Why did Hadrian dismiss Acilius Attianus from the Praetorian Guard?

Hadrian dismissed Acilius Attianus from the Praetorian Guard primarily to distance himself from the unpopular summary executions of four prominent senators.1

While Attianus, acting as prefect, had ordered these executions to eliminate perceived threats to Hadrian’s security before the emperor returned to Rome in 118, the act was viewed as a "bloody prelude" to the new regime. To maintain his own political standing and public image, Hadrian publicly affirmed that these actions were contrary to his will and laid the blame entirely on Attianus. By dismissing him, Hadrian was able to stabilize his position and court popular sentiment through public largesse and the cancellation of state debts.1