They will be thoroughly avenged by G-d, Himself. If I shall sharpen the flash of My sword and My Hand shall hold onto justice, for I will return vengeance to my oppressors and I will pay back My enemies. While Judaism encourages people to overlook, to forgive and forget the foibles of their fellows, this concept is not relevant when it comes to G-d, at least not within the above context.
Man is a responsible being.
If consequence of acts do not have to be taken seriously, then G-d would have had no reason to create and maintain the world as we know it. Instead, we would have all been 'born' in heaven. By design of the nature of Man, to fully derive benefit from doing good, Man has to be able to get into serious trouble for doing bad. G-d does not want anybody to be destroyed. Yet He lets people get themselves destroyed if they so desire.