Gio had rules when it came to killing humans. In order to keep himself on the right path, he promised to never kill a human he didn't believe was worthy of death.
It was hard to make that kind of judgement about someone he'd only known for mere seconds. As such, the rule had exceptions. Anyone who introduced themself by aiming for his life could be immediately classified as a target for execution.
After all, such scenarios weren't a matter of him playing judge and jury. It was an ordinary fight between Sequencers that could only have one victor. Just as Serene and Shiro were being forced to realize, there were times when one had to fight evil with evil without trying to justify every little thing.
Gio killed his opponent especially quickly, ignoring his code because he knew what kind of experience this was for his comrades.
