After a battle with Akainu reveals that both brothers were out of their depth, Ace saw the Marine gearing up to end Luffy's life.

Source: https://gamerant.com/anime-characters-died-heroically/

Akainu wins the battle at the Punk Hazard, he gains the right to command the full might of the Marine as its new Fleet Admiral.

Source: https://gamerant.com/one-piece-why-did-aokiji-resign-from-marines/

It wouldn't be shocking if someone managed to kill Akainu for everything he's done.

Source: https://gamerant.com/one-piece-characters-die-final-saga/

Realizing his underling's cowardice, Akainu did not hesitate before reducing the man to a smoldering pile of pulp.

Source: https://www.cbr.com/worst-things-one-piece-world-government-has-ever-done/

The two battled for ten days, and eventually Akainu came out on top.

Source: https://gamerant.com/one-piece-every-character-trained-monkey-d-garp/

To Akainu, anyone who did not directly benefit the World Government was almost as bad as the pirates themselves.

Source: https://www.cbr.com/worst-things-one-piece-world-government-has-ever-done/