because he has calmed down and now respects coby that he has Observation Haki while Akainu does notOkay, so we all know that during the Whitebeard War, Coby spoke out against the continuous killing and stood up to Akainu. Now, this visibly frustrated and infuriated Akainu, so he was gonna kill Coby till Shanks stopped him.
But what was stopping Akainu from killing Coby later, like during the timeskip? After all, as Fleet Admiral, Akainu could have executed Coby with no problem, so why didn't he?