Goodness is more of a destination than a process and is not based on any specific belief. This is because anyone who has a large range of the human virtues (some scholars believe there are 24) like justice, truth, fairness and integrity and manifests these in his actions and thoughts has a goodness. How advanced this goodness is depends on how unconditional it becomes. What this means is that we can't pick and chose when to be fair and to whom. In other words, we can't be fair to what fits our comfort zone and dismiss the rest. Overall, the more virtue a person uses to bring unconditional good to this world, the more he has climbed up the ladder of humanity. But an extremist does not think this way.
A religious extremist is a self righteous person gone too far. An example that comes to mind would be the born-again Christian who killed the abortion doctor or other examples of Muslims who kill the innocent in the name of defending Muslims or the Monks who destroy an orphanage, and then all the other historical stories in which people have killed the innocent in the name of religion.
There are many reasons behind why an average person may do something harmful to an innocent person while seeing it as a good deed. These reasons are multi-dimensional and to explain them in simple, tip-of-the iceberg terms will only add to the confusion, rather than bring meaningful understanding. But generally, a few of these reasons are the world's lack of a moderate, moral and fair role model, in addition to a lack of access to proper education in many places, lack of basic resources for many people, too much repression, autocratic rulers, closed minded and egocentric leaders, personality factors, family upbringing, a tarnished sense of pride, among many others.
Religious extremists, in most cases, are not psychopaths. The general characteristics of psychopaths are charm, no signs of irrational thinking, lack of remorse or shame, pathological egocentricity, being incapable of love, not having insight and no life plan. While some of these characteristics can be seen in people who turn extremist, many others are not. For example, while an extremist may numb himself to any feelings of guilt or shame in certain situations, in many cases and with the people he feels close to, he shows such feelings.
There may be a few general characteristics of people who turn into extremists:
1. They have a sense of absolutism: They have a distorted, nonconstructive and irrational thought that the truth, moral or aesthetic values are absolute, universal, set and unchangeable. They do not believe in change and diversity and are usually very low in their tolerance level.
2. They have a sense of righteousness: They usually think that they know the truth and no one else does. Their truth is very limited and based on outdated, contaminated and one-sided information. They usually don't even have enough knowledge about their own religion and only know the surface part.
3. They do confirmation bias: This is where one only brings in information that fits his thought process and dismisses anything else. Any other information, no matter how historically, scientifically and logically valid, will be disregarded.
4. They have a sense of knowing an ultimate meaning: They have a sense of a black and white thinking where the white is a limited definition of how life "should" be for all of us. There is no flexibility, no adaptability and no objectivity. You are either into this small and specific white zone or you are "the other."
5. They dehumanize whoever does not fit their view: They put other people's views inferior to theirs and dehumanize people whose views do not fit theirs. This gives them a sense that they have the right to kill, harm and destroy others. They also do the same to out-groups.
6. They idealize historic figures or stories: Such people usually idealize some figures in their belief system and stories attached to the past and want to fit the present and the future into that idealization.
7. They have an utter certainty that they are right: The objective mind of a rational person knows that at any time, there are so many things he does not know. But an extremist does not have such view and holds a distorted thought that he knows all the answers and has found the "truth" which is the only truth.
8. They have a sense of unwillingness to compromise: For such individuals, there is only one way and that is what matches their definition of truth. They are not willing to find common grounds with other people and cannot find win-win positions.
9. They have too much focus on the life after death: A religious extremist has too little focus on the importance of this life and what makes him feel fulfilled in it and is too attached to the concept of a "great" afterlife.
10. They have many psychological defenses: Such individuals have formed a number of psychological defenses so none of their internal feelings would be challenged.
At the end, targeting extremism is about targeting ideas more than individuals. There should be a significant concern about the rise of extremists with this populated world. This is our future as it defines whether humanity will have peace or a devastating conflict that will cost all of us tremendously. If there is any war, it should be a collective and universal war against extremist through education, knowledge and information.
Some of the ways to go against extremism are to increase your own knowledge and to speak your truth against them. For example, if you are in a gathering, organization, institution or in any setting that preaches an extremist view that encourages hate, divides us, creates tension and conflict based on beliefs and caters anger and frustration based on subjective feelings about how things "should" be rather than an objective and rational share of information, then maybe it's a good time to speak up. Don't expose your children to situations that create such invalid internal feelings toward a group of people and encourage them to increase their awareness by exploration and information. In addition, encourage them to be reasonably skeptical but at the same time open minded.