How did this transformation of Satan happen? 

Consider what could happen to a human being if they were extremely beautiful, intelligent, and influential in world business and politics. 

It is needless to say that this person would become intensely proud of his/her accomplishments and even desire greater power, wealth, and prestige. Satan was considered the most beautiful angel created and was also responsible for leading the worship in Heaven.

Unfortunately, Satan himself was responsible for his transformation into the devil by the craving into his pride, greed, arrogance, and self-seeking ambition to be worshiped equally if not even greater than God.

Q. Why does God not destroy Satan if He is omnipotent?

Because God has not yet worked out His sovereign purposes through Satan. Though Satan himself is evil, God still accomplishes His purposes of good through him.

One example of this is where the messenger of Satan that was sent to buffet Paul worked only for the good of Paul. He kept Paul from being exalted above measure.

2 Corinthians 12:7-10 (NKJV)

7 And lest I should be exalted above measure by the abundance of the revelations, a thorn in the flesh was given to me, a messenger of Satan to buffet me, lest I be exalted above measure. 

8 Concerning this thing I pleaded with the Lord three times that it might depart from me. 

9 And He said to me, “My grace is sufficient for you, for My strength is made perfect in weakness.” Therefore most gladly I will rather boast in my infirmities, that the power of Christ may rest upon me. 

10 Therefore I take pleasure in infirmities, in reproaches, in needs, in persecutions, in distresses, for Christ’s sake. For when I am weak, then I am strong.

Eventually, yes, God will take satan out, but until then, satan has been allowed to surface around, but only as much as God himself will allow him to.