The mechanical properties of body-centered cubic metals at low temperatures are strongly influenced by the solute atoms, which could be relatively complex as either hardening or softening effects would be introduced depending on the solute concentration and temperature. One major impact is that solute atoms can affect both kink formation and motion during screw dislocation gliding, which plays an important role in plastic behavior of bcc metals. In this study, atomistic simulations are conducted on the carbon-affected kinking of a screw dislocation in iron using the nudged elastic band method. When kink nucleates alongside a carbon atom, the kink formation energy decreases as the carbon transits to a stronger binding site, and vice versa. When a single kink meets the carbon during propagation, the sideward motion of the kink is impeded. With regard to the different temperatures and solute concentrations, the softening and hardening effects induced by carbon solutes in iron can be explained by the complex atomistic process.