Let's think thru this verse logically.
Matthew 10:28 And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell.
Here we see that some are able to kill the body, but not able to kill the soul also. We're told not to fear them. Then were told..rather fear him which is able to destroy both soul and body in hell. Clearly this passage indicated, that not only is God able to destroy the body, just like the ones we're not to fear can as well, but that He can also do what they can't do, and that is to destroy the soul.
Speaking of destroy, let's see how that is used in another passage, in order to try and determine the sense in this passage.
Matthew 2:13 And when they were departed, behold, the angel of the Lord appeareth to Joseph in a dream, saying, Arise, and take the young child and his mother, and flee into Egypt, and be thou there until I bring thee word: for Herod will seek the young child to destroy him.
This same Greek word is used here, in this part...for Herod will seek the young child to destroy him. So what exactly should we conclude that Herod was seeking to do to the young child if found? Wouldn't it be safe to conclude he would kill Him? So wouldn't the sense for destroy in that passage mean to kill?
With that in mind, the same sense should apply to Matt 10:28. So IOW, it should be understood like such.
Matthew 10:28 And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to KILL both soul and body in hell.
Even tho different Greek words were used in this passage for kill and destroy, we at least know what sense destroy is meaning. And on top of that, the latter shows that only God can destroy(kill) both body and soul, and that man can only kill the body. So now the question is, what does it mean to kill a body and a soul? How can a soul be killed, yet still be alive? It would be like asking, how can a body be physically killed by someone and then still be alive? The body would be dead. But if the soul is still alive after God has destroyed(killed) it in hell, then so should someone's body still be alive after someone has physically killed it in this age. We both know that that's illogical, in regards to the latter. So if it's illogical in regards to the latter, it must then be illogical in the former as well.