This article is going to briefly answer the question that "why did God the Almighty create Satan" whereas he is the most gracious and merciful towards mankind.
Some may assume that it was God, the Almighty, who created Satan (Evil) and He granted him control over the mankind to misguide them. Thus, God, the Exalted, is the ill-wisher of the mankind as his creatures.
But it is categorically wrong to attribute such things to God, the almighty, because according to the Islamic teachings, God did not create Satan as Satan; rather, He created him as angel, and Satan was one of the Angels and worshippers of God, the Almighty, for thousands of years. [1] It was Satan, himself, who decided not to obey Allah (J) with his own free will; consequently, he was distanced from Allah's mercy and blessings.
Therefore, Allah is not the creator of Satan as Satan to be called as ill-wisher of mankind; rather, He is the well-wisher and gracious creating angels and human beings. Since Satan has not been granted control over the mankind [2] and he merely calls upon man to follow him [3] whereas man is free to choose the right path; [4] Satan is mercy helping mankind to progress and perfect them.
Of course, on those people who follow Satan listening to him, Satan has the control and they will be misguided if they continue following the way of Satan. As it is stated in the Holy Quran that:
His authority is only over those who befriend him and those who make him a partner [of Allah]. [5]
Notes:
[1] Nahjul-Balaghah, the sermon of ‘Qaasi’ah, number. 234.
[2] Nahl: 99. إِنَّهُ لَيْسَ لَهُ سُلْطَانٌ عَلَى الَّذِينَ آمَنُوا وَعَلَى رَبِّهِمْ يَتَوَكَّلُونَ
[3] Ibrahim: 22. وَمَا كَانَ لِيَ عَلَيْكُمْ مِنْ سُلْطَانٍ إِلَّا أَنْ دَعَوْتُكُمْ
[4] Insaan: 3. إِنَّا هَدَيْنَاهُ السَّبِيلَ إِمَّا شَاكِرًا وَإِمَّا كَفُورًا
[5] Nahl: 100.إِنَّمَا سُلْطَانُهُ عَلَى الَّذِينَ يَتَوَلَّوْنَهُ