Noah is portrayed as a gentle man, a kind, compassionate individual in a darkening world. He's given terrible orders from above. He obeys them because he sees no other choice. It hardens him, pushes him to the brink of insanity.
A man who wouldn't harm a single living thing is charged with assisting in the destruction of all. His very complicity creates psychopathy. He doesn't merely build a sanctuary and sail it, he fights off all he deems unworthy to survive.
Those doomed to obliteration do not go quietly to their Fate. They fight with everything they have to find physical salvation. Noah condemns them all, with unflinching righteousness, to the abyss. Extinction on a scale so massive, that it can barely be comprehended.
Of course, it's easy to see Noah as a religious fanatic, so lost in his own interpretation of divine revelation that all other considerations pale into insignificance. But there's another telling here too.
It's the story of everyone who ever blindly followed directives from their superiors. A fact that is underscored when Noah sees humanity reduced to silhouetted figures of soldiers flashing through the ages. Brother against brother finds expression in Cain and Abel, but that is merely a metaphor for all war. Nation against nation. Religion against religion. Race, ideology, all that ever caused one human being to feel that another had to die for their perceived difference.
In Noah's marionette display, those soldiers weren't merely contemporary. They wore the uniforms of warriors throughout the ages. They exist right now.
And it's that two-dimensional vision which presented humanity at its darkest edge. It was the petty violence that pushed Noah into supporting an act of absolute violence - the annihilation of all.