Jesus said:

Mat 10:28  And do not fear those who kill the body [soma] but cannot kill the soul [psuche]. But rather fear Him who is able to destroy both soul [psuche] and body [soma] in hell [gehenna].

A similar statement is also found in Luke.

Luk 12:4  “And I say to you, My friends, do not be afraid of those who kill the body [soma], and after that have no more that they can do.
Luk 12:5  But I will show you whom you should fear: Fear Him who, after He has killed, has power to cast into hell [gehenna]; yes, I say to you, fear Him!

The meaning of what Jesus said is made clear when the Greek words translated as “body”, “soul”, and “hell” are properly understood.

Soma

The English word “body” is translated from the Greek word soma.  According to Thayer and Strong soma has a very wide application (much like the English word “body”), but in particular it can mean the physical body of a person, dead or alive.

Here is an example of where soma refers to a living body.

Mat 6:25  “Therefore I say to you, do not worry about your life, what you will eat or what you will drink; nor about your body [soma], what you will put on. Is not life more than food and the body [soma] more than clothing?

Here is an example of where soma refers to a dead body.

Mat 27:58  This man went to Pilate and asked for the body [soma] of Jesus. Then Pilate commanded the body [soma] to be given to him.

Psuche

The English word “soul” is translated from the Greek word psuche.  According to Thayer and Strong psuche means breath, soul, or life.  It is often translated “life”.  Here is an example.

Mat 10:39  He who finds his life [psuche] will lose it, and he who loses his life [psuche] for My sake will find it.

From this verse it is clear that psuche is not something immortal, as it can be lost.

Gehenna

The English word “hell” in these verses is translated from the Greek gehenna.  According to Thayer and Strong gehenna is of Hebrew origin, meaning “valley of hinnom”, a valley south of Jerusalem where rubbish and dead animals were burned.  This burning “hell” is a reference to the lake of fire where those who refuse to repent will be cast.

Rev 20:14  Then Death and Hades were cast into the lake of fire. This is the second death.
Rev 20:15  And anyone not found written in the Book of Life was cast into the lake of fire.

Rev 21:8  But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death.”

This lake of fire is the second death, Revelation 20:14.  The first death is what all humans experience when we die.  In fact, it is appointed (part of God’s plan) that we all die once.

Heb 9:27  And as it is appointed for men to die once, but after this the judgment,

The first death is temporary.  Some rise (at the first resurrection) from this first death to everlasting life, Revelation 20:4-5. 

Rev 20:4  And I saw thrones, and they sat on them, and judgment was committed to them. Then I saw the souls of those who had been beheaded for their witness to Jesus and for the word of God, who had not worshiped the beast or his image, and had not received his mark on their foreheads or on their hands. And they lived and reigned with Christ for a thousand years.
Rev 20:5  But the rest of the dead did not live again until the thousand years were finished. This is the first resurrection.

And some rise (at the second resurrection) from this first death to judging, Revelation 20:12-15. 

Rev 20:12  And I saw the dead, small and great, standing before God, and books were opened. And another book was opened, which is the Book of Life. And the dead were judged according to their works, by the things which were written in the books.
Rev 20:13  The sea gave up the dead who were in it, and Death and Hades delivered up the dead who were in them. And they were judged, each one according to his works.
Rev 20:14  Then Death and Hades were cast into the lake of fire. This is the second death.
Rev 20:15  And anyone not found written in the Book of Life was cast into the lake of fire.

The second death, Revelation 20:14, is what those who enter this lake experience; they suffer permanent death.   They suffer complete destruction; a final punishment that is everlasting, eternal, permanent!  It is not the punishing that is everlasting but rather the punishment that is everlasting. 

Conclusion

Matthew 10:28 and Luke 12:4-5 now become clear.

Man can kill a physical life, which is the first death, Hebrews 9:27.  However, man cannot destroy someone so that they will never live again, which is the second death, Revelation 20:14.  God, on the other hand, has the power to do both. 

Mat 10:28  And do not fear those who kill the body [soma] (physical life) but cannot kill the soul [psuche] (potential for eternal life). But rather fear Him who is able to destroy both soul [psuche] (potential for eternal life) and body [soma] (physical life) in hell [gehenna] (the lake of fire).

Luk 12:4  “And I say to you, My friends, do not be afraid of those who kill the body [soma] (physical life), and after that have no more that they can do.
Luk 12:5  But I will show you whom you should fear: Fear Him who, after He has killed, has power to cast into hell [gehenna] (the lake of fire); yes, I say to you, fear Him!

If men kill us with the first death, it is only temporary.  If God kills us with the second death, it will be forever.