Satan is only 'king' of this age, or this era on earth - it's not permanent. Jesus defeated death, and saved all who believe in Him from certain doom. That was a massive defeat for Satan. But that doesn't mean Satan is gone. We are still here and still fallen men, and Satan is alive and well on planet earth. Revelation is when it all comes to a head. Until then, the war rages on.

https://www.gotquestions.org/Satan-god-world.html