In social decision-making games, uninvolved third parties usually severely punish norm violators, even though the punishment is costly for them. For this irrational behavior, the conflict caused by punishment satisfaction and monetary loss is obvious. In the present study, 18 participants observed a Dictator Game and were asked about their willingness to incur some cost to change the offers by reducing the dictator’s money. A response-locked event-related potential (ERP) component, the error negativity or error-related negativity (Ne/ERN), which is evoked by error or conflict, was analyzed to investigate whether a trade-off between irrational punishment and rational private benefit occurred in the brain responses of third parties. We examined the effect of the choice type (“to change the offer” or “not to change the offer”) and levels of unfairness (90:10 and 70:30) on Ne/ERN amplitudes. The results indicated that there was an ERN effect for unfair offers as Ne/ERN amplitudes were more negative for not to change the offer choices than for to change the offer choices, which suggested that participants encountered more conflict when they did not change unfair offers. Furthermore, it was implied that altruistic punishment, rather than rational utilitarianism, might be the prepotent tendency for humans that is involved in the early stage of decision-making.