It is not a matter of how serious his mistake is. A mistake is always a mistake.
But what is important here is that his belief made that mistake become a burden. He would have felt comforted if he had believed that everything that happened is just the prearrangement, the predestination of the Creator and that he didn’t have any capability to change those things since the past can’t be changed. We are not debating whether this is a right belief or not but we emphasize the effect such a belief would have on the person’s state of mind. It is also not considered here whether the relief is a short or long term one.