Luffy managed to get Ace free from the execution platform and they run to savety. Till they got separated and Admiral Akainu is holding Luffy hostage. And threatens to kill Luffy if he doesn't allow himself to be executed

Ace looks around and sees that you are hold as hostage too by Kizaru

Akainu: so Fire fist! Your choice! Who gets killed? You? Your little brother? Or your Love~?

he says the last part in a amusing tone

Kizaru: Make a choice! Or both die!

Ace is to shocked to decide