Before the One Piece time skip, Sakazuki, formerly known as Admiral Akainu, was a force of the Marines to be reckoned with. During the War of the Best in Marineford, he starred the naval offensive against the disintegrating Whitebeard Pirates in a persistent endeavor to murder Portgas D. Ace and Monkey D. Luffy. He confronted Whitebeard several times and demonstrated a ferocious determination to destroy every pirate in his way. Sakazuki's unwavering determination, backed by his belief in 'Absolute Justice,' made him one of the most fearsome enemies on the Marineford battlefield.
After the war's conclusion, Sakazuki led the navy warships in pursuit of Edward D. Teach and battled against Kuzan for supremacy of the navy leadership. In his victory, he was promoted to Fleet Admiral and took command of the entire navy force. It would not be a stretch to assume Sakazuki would take the reins of his new position and begin a crusade of absolute annihilation of the pirates throughout the world -- yet no such act has come to pass. In fact, it appeared as if he had become more complacent.
Sakazuki, the Admired Officer Worker from One Piece
The overly simple answer for why Sakazuki held back in his wish for a pirate-free world was due to the lack of freedom the Fleet Admiral role is forced to endure. Admirals could only be sent into the field at the behest of the Celestial Dragons or in major military operations. However, as Fleet Admiral, it could be assumed he would have the authority to launch those operations as he saw fit. Instead, Sakazuki has remained either in his office -- planted behind a desk -- or arguing with the Five Elders.
Due to the high rank the Fleet Admiral holds, there are only two positions he answers to, and a third he is likely unaware of: the Commander-in-Chief, the Five Elders and the mysterious ruler, Im. The relationship between Sakazuki and the Five Elders appears to involve mistrust and strain, possibly implying he is not as trusted to be privy to secret information that Sengoku may have known. Considering this fact, it could be by the authority of the Five Elders that Sakazuki is shackled to his desk and unable to fulfill his destructive desires.
Sakazuki knows to take responsibility for the damage he causes
Sakazuki was once known as the Mad Dog of the Marines. Now that the responsibility of the entire world's balance is on his shoulders as the face of the Marines, the repercussions of any violent actions are all the more real. He trusted Fujitora to hunt down the Straw Hats, but is not ignorant enough to expect Fujitora to have the same drive as himself; he also warned the other two Admirals not to engage in hostilities or stir up trouble. Sakazuki is holding the Marines back, unwilling or unable to use them for the greatest level of impact possible.
Perhaps complacent is not the exact term for Sakazuki’s actions; it is more of a forced complacency. As it is implied that his and the Five Elders' ideals do not align, the friction between the two factions within the World Government seems to cause internal complications that keep Sakazuki restrained.
CP agents are performing operations where Sakazuki is told to stand aside, SWORD is working secretly within the marine forces, and One Piece's pirate world is turning on its head with dramatic shifts in power. Sakazuki is not trusted by the Five Elders as the Fleet Admiral, and he knows it.