Jesus himself explained it , " ....  they know not what they do ."   And asked his Father to forgive us .
Your take on humanity presupposes human perfection .  The fallen angels are said to have totally comprehended their act of rebellion  but not us . 
Our condition is a mitigating circumstance whether you like it or not .   Not an excuse ,  a cause .
Humans can and do sin but probably not as frequently or seriously as some might think  .
The focus is best placed on how are we to be healed of the woundedness which grips us  rather than how much punishment needs to be meted out  .
God , even when we were his enemies loved us .  God so loved the world that he gave his only begotten Son so that the world might be saved by him .