Song of the wings

"Never question, just act." That's what all the assassins working for the brothers, the angels who came down on earth to help humanity, were told. Same applies for Isabel, a young woman who was raised by the brothers and chosen personally by them to become and assassin. Her most recent mission, to take out a 'problem' ,as they called people opposing the greater change for the better, when angels would rule. However, her assistant is a rookie angel, Rin.
But... What if change isn't always the best choice..? What is people started asking themselves 'useless' question...? What if... humanity is endangered...?