blood of madam giselle ending

If Isaac has been the killer all along (and not just a rushed way to end the series), what was his motive to kill innocent people? Or was there never a motive and he was just blood thirsty all along and hid it from Giselle? How could Giselle end him without a second thought? She loved Isaac so much yet she shot him straight away. I loved Isaac. the power of love lirik