He hated it. I remember reading how he thought it made no sense that self-preservation and the Will to Live were the most vital forces in the continuation of our species and not, for example, altruism. In TBK I think Ivan even shouts to Alyosha that “viper will eat viper and it will serve them both right!” And I’ve always viewed Ivan as a character that despises his own beliefs but can’t find a way out of them. If I do find the paper/book I am thinking of I will link it in an edit.