The funniest thing is seeing people IRL being swayed by the villains' arguments, but then....I guess we just have to look at how the French Revolution turned into The Reign of Terror, and how every similar, progressive revolution has ended the same. Humanity is definitely capable of following path of "good intentions" all of the way to Hell, over and over and over again, never learning.
Nothing these murderers say will absolve them of guilt. Things not being absolutely perfect just isn't the winning argument that people with excess, misplaced empathy think it is. It just isn't.