There is no such thing as absolute justice. Ethics is based on moral principles, which were relative and subjective. Also it could be argued that upholding the lives of innocents should be the number one rule of anything approaching absolute justice so Aikanu would be s huge hypocrite
Correct me if I'm wrong, but the marines led under Akainu are all followers of Absolute Justice.
I'm pretty sure that scene was used to demonstrate that, at the very least, a significant faction of Marines operate under Absolute Justice. As such, Absolute justice is either a social construct that the marines base their lives around or an actual legal principle that they are obliged to enforce as marines.
Agree with it or not, that is literally how their ethical code is defined. As such, Akainu's "I will kill as many people as I have to as long as you are one of them" conduct is ethical.
I'm not trying to debate that he's right or that makes him good or anything like that. It's just how the words you are using work.