All living things, be they human, animal, or vegetative, are fundamentally selfish. This is how evolution works.
In social animals there is an inherent conflict between the first order "selfish" where we do for our selves, second order "selfish" where we do for our family, third order "selfish" where we do for our tribe, and fourth order "selfish" where we do for our species. A "dark side" is only an ideological construct to pull us closer to that fourth order ideal and only has power because we have become a species that embraces specialization and requires some level of support from all four orders.
Ask a flower or a coyote what the concept of a "dark side" means and you will get very different perspective.