The Marine hero Garp always wanted his rebellious grandson Luffy to become a Marine, and each time fans saw Koby's progression, he seemed more like the Marine that Garp always wanted his grandson ...
Then he failed to convince Ace to become a marine like him. Finally, Garp was unable to stop Luffy from pursuing his dream of becoming a pirate. All these things likely weighed on his mind ...
if Garp was two years younger, the fight may have been a different story. Luffy and Blackbeard's dynamic in One Piece provides an interesting twist to other hero-villain duos. As for Akainu ...