Why do people justify their immoral behavior as being moral?

The human heart, while capable of great love and compassion, can also be ‘treacherous and desperate.’ (Jeremiah 17:9) This sad reality is reflected in the world’s changing moral landscape—a trend the Bible foretold. “In the last days,” it said long ago, “people will be selfish, greedy, boastful, and conceited,” as well as “unkind [and] violent.” Also, “they will hate the good” and “love pleasure rather than God.”—2 Timothy 3:1-5, Good News Translation.

Those realities should move us to question our own heart, not blindly trust it! Indeed, the Bible frankly states: “He that is trusting in his own heart is stupid.” (Proverbs 28:26) Like a compass, our heart needs to be calibrated with sound values if it is to serve us well. Where can we find such values? Many look to the Bible itself, appreciating both its wisdom and its candor.

Morality is only a matter of perspective. What you may deem 'moral' is only a learned personal preference that is a product of your relative environment; and, needs no 'justification'. Do not be swayed by the judgement of others; as, the most harsh judge you will ever face is...the "Self".

There are different type of spirits with different characters in this world. There will be always such characters until the earth is alive.

Human beings are ruled by spirits but humans must learn to control and rule the spirits. Spirits possess people according to the mental state of the human beings and keep them in the same state until they are rejected and replaced by another. This is an all time phenomena occurring in all humans. Human beings are always under the influence of spirits. Certain situations and circumstances causes the possession of certain type of spirits. Often thinking and talking about bad subjects or bad imaginations even for fun can take serious turns on one who think and talk. All spirits (good and bad) have the dominating character. They can take you to the extent of sacrifice or suicide.

All of your activities, talks and even thoughts determine what kind of spirits you will have for your life. Only these spirits are the reason for the good and bad happening in your life. Generally spirits determine your destiny from your daily activities, talks and thoughts too. All knowledge, skills, emotions, feelings, interests and everything are spirits. Even thoughts are not your own. When you want to take a decision on a subject, one after another they think and you just listen and choose or reject the ideas which they transmit to your mind through your brain in the form of thoughts. A human being during his/her lifetime is living with many spirits which have joined one by one since birth. All human beings are just unprogrammed robots made of flesh, bones, tissues etc., by birth and toys of the spirits for their games.

In order for a person to do things without guilt, he has to first convince himself that the things he does are acceptable and right. Its a matter of conscience

Men are quite adept at rationalizing and excusing negative behaviours when it serves their own needs.

A popular quote "The end justifies the means."

Everyone thinks their actions are sacred and other people's are evil.

If you want to know about how and why people self-justify examine your own behavior and if you find you are free from sin and self-deception you aren't really looking.

Because they likely view it as moral. Ethics and morality is an individual's perspective, what you view as immoral might not be what they see it as.

Example?

They want to have a clear conscience and feel good about themselves. They want to have an excuse to do immoral and irresponsible things without having to feel any guilt for it.

Simple, he justifies his immorality because he is immoral to the core.

Human nature.