Oram felt responsible for the failure of the Covenant. Using this built up frustration he kills the neomorph, confronts David, and follows him for answers. If he made one mistake it was following David by himself, but David wasn't going to let those eggs sit there with viable hosts running around on his planet for the first time in 10 years. He would have kidnapped someone to see what it would become, I can assure you. Oram was just easy pickings because he was so emotional and naive. Does it make him stupid? I would say the adrenaline and emotions clouded his judgement. It's amazing what duress can do to the mind. That's why I don't fault characters making bad decisions mid action sequence or after seeing someone brutally murdered. When they are not under duress and making these types of decisions, then yes, I'd call it stupid.