According to the Final Fantasy XII manga, Gabranth was the one who killed King Raminas.
- —End of spoiler section
What...? Gabranth/Noah is the only person who looks like Basch, and Vaan and Ondore both blame Basch for Raminas's death. In addition, this proves it without any need for the manga:
What if a Judge killed the King, not the Captain? That would explain everything, wouldn’t it?
- —Some guy in the Resistance