There is war, famine, hatred and evil in the world because all animals get into situations where they cannot behave or live in peace and with the resources that are available to them.
The easiest way to explain why there is war is to look at the causes of war. There may be scarce resources and well organized and equipped groups that control those resources. Another group needs or wants them, and diplomacy does not work. As a result, one group attempts to decimate, incorporate or evict the other. The famous Meerkats demonstrated that animals who have limited places to burrow out housing, to get water and food and to fend off non-Meerkat predators caused various groups to stay in battle with each other. Over time, hate, aggression and rejection on sight became the norm.
There may be some who are crazy or greedy or who seek retribution on an ongoing "tit for tat" basis. Those who have the position, charisma or power to make others do the fighting with or for them will go for land, resources, control of populations or supremacy over regions and religions. Sometimes there are strategic or profitable reasons for wars, such as the USSR's goal of having a warm water port nearest to their capitals. Sometimes wars are responses to threats or are retaliations for attacks of a great nature.
The simplest way to explain why there is racism is to look at any animal's ability to recognize their own tribe, group, family or genotype. It is instinctive to distrust or to reject those who are different for a host of reasons. Amazingly, when humans are required to work with other humans to survive, or during great endeavors, they cease to focus on race, gender or other physical and easily observable differences. Many will make the remark "I completely forgot that that person was a...."
The simplest way to explain why there are large groups who hate, express racism and do evil as a result, is that people are taught or convinced of hateful, racist or evil ideologies and dogmatic beliefs. When they isolate and segregate, they reinforce and pass the ideologies and warped belief systems on to the generations.
When there is a need to respond to an attack, the most powerful tool for getting support from the populace is to encourage and amplify the rage and contempt for the enemy or the attacker. Thus, we are less inclined to reject racial and ethnic slurs and other expressions of nationalistic hatred and aggression. Sadly, these ethnic, xenophobic and racial hatreds are believed to be the necessary emotional and behaviorial preparations for a nation or group that needs enough support and public aggression to go to war.
There is famine because of complete disrespect for the world's habitats for food producing plants and animals. The Western systems of developed countries ruined vast parts of the world with the idea of "the bigger the better". Instead of hunting, gathering and small farms that sustained families, leaving no real impact on the environment, there had to be overgrazing, which led to desertification. Instead of husbanding sustainable forests and doing restoration of strip mines, there was deforestation and stripping of the fertile soil layers at a magnitude that was never done before, leaving sterile land behind. Water diversion and damming alone is responsible for more devastation and lost fertile land than any other endeavor.
The easy way to explain pure evil lies in a basic understanding of the psychopath and the sociopath. It has been said that the psychopath is born wrong while the sociopath is made wrong. The sociopath can be treated, if caught in early childhood. The psychopath has mechanical issues with the brain and is born with them. The well controlled psycopath or sociopath is very able to function highly in society, meticulously concealing and compartmentalizing their evil and wrong deeds while completely fooling everyone else who knows them. As a result, they may never be identified or caught.
We call psychopaths "monsters" because, many times, the individual has no past trauma or lack of opportunities in life that could explain the damage, harm and suffering that they cause. They truly have some internal satisfaction that they desire to get, and they show no empathy, remorse or respect for others in getting that satisfaction.
We try to control and rehabilitate sociopaths before they become psychopaths, because their makeup originates with childhood trauma, spoiling or neglect, and many can be treated. But we often fail to identify or fix them until they get into great enough trouble to go into the justice system. Even animals have psychopaths and sociopaths who might kill and eat infants or attack their own community members without a reasonable cause, such as battling for supremacy.
Finally, we define evil either because we decide that something is evil or because someone convinces us that something is evil. People who want to tell everyone else what is evil are evil. People who justify evil with the words "necessary evil" are evil. Psychopaths, some drug addicts, criminals and sociopaths are inclined to cause harm, destroy, cause suffering and to disrupt life for others just for kicks, and that is evil.
Evil, then, is whatever we believe it to be. And humans will hate, kill and be killed in their ongoing battles when threatened by something that they believe is evil. And that is why there is so much of the wrong kinds of war, famine and hate in the world.