The declaration is meant to free her of any responsibility whatsoever. It is meant to build her defenses in the event that she is charged before the international criminal court for crimes against humanity. Thus, even without a rebellion she issued the declaration. It is for her self-preservation and our laws and the Constitution can all go to hell for all she cares.
The accused masterminds were her loyal party men, the other accused were armed by and employed by her government. She is worried that liability will be pinned on her and so to save her own skin, she needs to sacrifice the Ampatuans and all others implicated in the heinous offense.
She cares little about local public opinion. It is the opinion of the international community that matters dearly to her. She dreads being hailed to the international criminal court for crimes against humanity. The Martial Law declaration is meant to appease international indignation and does not look to a nationwide Martial Law declaration in the future.
Martial Law was meant to save her own skin from possible liability under both Philippine law and international criminal law.
She isn't out to save the Ampatuans. She is out to save herself. For the cause of her own survival, no one in indispensable.