Quran says the human beings have been innately given a sense of right or wrong – sort of an innate moral compass.
وَنَفْسٍ وَمَا سَوَّاهَا فَأَلْهَمَهَا فُجُورَهَا وَتَقْوَاهَا قَدْ أَفْلَحَ مَنْ زَكَّاهَا وَقَدْ خَابَ مَنْ دَسَّاهَا
And the soul bears witness and the perfection given to it, then [God] inspired it with its evil and its good that he succeeded who purified it and he failed who corrupted it. (91:7-10)
A person is not mere body and intellect. He is also a moral being. This means that the ability to distinguish good from evil and the appreciation that good is good and evil is evil is innately ordained in him. In some other verses of the Quran, this aspect is referred to as: إِنَّا هَدَيْنَاهُ السَّبِيلَ (We inspired in him the way of good and evil, (76:3)) and وَهَدَيْنَاهُ النَّجْدَيْنِ (have We not shown him both paths? (90:10)). This appreciation of good from evil is a universal reality. Thus when the most evil of persons does something wrong, he tries to hide it in the first phase. When Adam’s son, Cain, tried to hide the body of his brother, Abel, after killing him, it was because he knew that he had committed a sin. Same is the case with good. A person loves what is good and respects and reveres it.
Iblees is a title (لقب ) given to Azazeel (عزازیل) who belonged to the community of Jinns. His rebellion made him a representative of Evil and hence had been used ever since as an alternative of evil or manifestation of extreme evil. He had free will just like Angels to either obey the command of God or disobey Him, but he chose to disobey and hence was given the title of Iblees.
As stated earlier, the Satan is not the primary source of evil. A human being knows about good and evil. There are different factors which persuade or attract a human to commit evil or sin. These factors include wishes, desires, sexual tendency and emotions etc. Sometimes, Satan or evil jinns also use these factors to persuade a human being into committing evil. It is ultimately a human being who decides his path and even in the absence of Satan, these factors listed above would still be there to make a person fall into sinful activities or to live a life of evil.