I think the loss of belief in some sort of ultimate judgment did give a certain type of person who is restrained by a fear of punishment rather than any ethical commitment or sympathies, more of a free rein.
And to deprive someone therefore of a meagre 90 or 100 years, the only glimpse they'll have out of potentially hundreds of billions of years underscores the preciousness of life and why it should not be taken.
I will give you that there are surely some who can say fear of gods kept them from killing but in fairness surely there are some who were set free from those gods and realized they didn't have to persecute others just because a religious doctrine says, even if their sympathies said otherwise to the doctrine.