Depending upon the circumstance, carbon dioxide can be helpful or harmful. Plants inhale carbon dioxide and exhale oxygen. So, in this instance, carbon dioxide is necessary for the health of plants.
Carbon dioxide is produced as a byproduct of respiration by most animal on earth, such as human beings. We inhale oxygen, which is exchanged in the lungs for carbon dioxide to be exhaled as waste. Inhaling large amounts of carbon dioxide can lead to asphyxia (or asphyxiation), insufficient oxygen in the blood stream... which can lead to death.