Stoicism is a strong concept. It's been a part of Hinduism for ages now. I did my research for a research paper and found that a clan called "Kshatriyas" were fighters and Stoics.
I think your character can be an atheist whilst being a utilitarian. I'd like to see a character who'd free from religion, culture, and a true humanist.
I think you should write your character as a person who follows Humanism. MAke if your thing.