All those people died primarily because of asphyxiation. Since CO2 is heavier than O2 and N2 it displaces air near the ground, starving the person from oxygen. Note that this usually happens at CO2 concentrations above 5% or 50,000 ppm, which more than a thousand times the current atmospheric level of 400 ppm. SO2, on the other hand can be dangerous at concentrations of even 5 ppm.