So why doesn't Gd destroy him then?
There is divine wisdom at play here with the creation of Satan.
If Satan was to be destroyed then how would we i.e. Humans progress spiritually which we have the potential to and reach great heights in the Unseen realm i.e. the Hereafter?
If we fight Satan and his ilk and do not fall for the temptations he invites us to, then every time we struggle against the influence we come closer to Allah (swt) and this is what Allah (swt) created us for. He in His Kindness wants us to progress so that we fulfill the purpose He created us for in this world so we could have the best life forever in the Hereafter.
In order for us to reach great heights and bring out the actual potential within us we have to fight the evil tendencies or evil commanding soul (Nafs) within us, whom Satan tries to invite towards all kinds of Haraams (illegal things as declared by Allah).
However, Allah (swt) in His Eternal Mercy has not given Satan the power to control weak creatures like us, he has no power over us, all he can do is 'Invite us' or our evil commanding soul (Nafs) and if we obey our evil commanding souls then we ourselves our led towards our own ruin, but every time we reject the temptation it creates within us to do something wrong, we achieve 'rewards' and those rewards are ultimate spiritual development the effect of which we see in our life in this world in the face of increased guidance, faith and nearness to Allah (swt) and in the Hereafter it is seen in the state of lofty positions in Paradise.
In the Quran Allah reminds us:
(14:22)
And Satan will say when the matter has been concluded, "Indeed, Allah had promised you the promise of truth. And I promised you, but I betrayed you.
But I had no authority over you except that I invited you, and you responded to me. So do not blame me; but blame yourselves. I cannot be called to your aid, nor can you be called to my aid. Indeed, I deny your association of me [with Allah ] before. Indeed, for the wrongdoers is a painful punishment."
Satan will say on the Day of Judgment to blame your self (Anfus or plural of Nafs) i.e. the evil commanding souls that you should have resisted against and not submitted to when I was inviting it as he admits Allah (swt) had given him no power over us.
The creation of Satan could also be understood in the following good example shared today by an Imam was that you walk in to a house and the dog of that house tries to chase you and runs after you wherever you go and once you see the owner of the house you call out to him for help and he protects you from that dog because only he knows how to control it because he trained it. Similarly Satan is after you and me and trying to take us away from Allah (swt), attack us and ruin us and the only way to save ourselves is by seeking refuge in the One Who created it i.e. Al Mighty Creator. By doing so we come in to Allah's Protection and remain in this for the rest of our lives safe from all evil forces in this world. We pass away like this in to the next realm and then gain eternal blessings and never ending rewards from Allah (swt) which we cannot even imagine.
So destroying Satan downplays the purpose of our existence.
My advise would be to you, to read what Allah (swt) has said in the Quran and follow His Word and come to Islam, before its too late.