In Philosophy, there is no proof or data that you base any answer on, rather your morality and ethics come into play. The first question we were asked by Dr. Baron was if we would save the buss full of children on kill one innocent life to save many. Instances like this one tend to be the hardest decisions that one would be able to make in their life. There is no "right" or "wrong" thing to do at this point in time and walking away is not an option.
In our modern day society, there is an excess of evil. It ranges from petty acts to extreme acts of terrorism. There is no true explanation as to why evil exists in our society other than to say that people can be intrinsically and extrinsically motivated to do just about anything.
In The Hunger Games, the whole series revolves around the hunger games and in essence the killing of 11 innocent children for the person in power, President Snow, to prove a point that there are punishments when laws are not adhered to.
In the series, Katniss is seen to be adhering to the principle of Utilitarianism. This is the concept that makes the most people happy and minimizes pain. Katniss is a prime example of this when she reluctantly agrees to become the Mockingjay. Peeta is also seen upholding this principle when he volunteers as tribute in Catching Fire in place of Haymitch. These instances do not coincide with the principle of 'the nature of evil" because these people are shown sacrificing themselves for the greater good.