Jehovah's Witnesses teach Jesus was created and he's "a god" according to John 1:1 in their NWT. Yet the Bible teaches all men are sinners?

Jehovah's Witnesses believe Jesus was created as the Archangel Michael. So you might as well ask how Trinitarians reconcile this with Jesus being 'fully man'. They are similar beliefs in that there is literal pre-existence as a 'logos-being' (Archangel Michael, Second Person of the Trinity) who then 'becomes flesh'. What's good for the goose ...

But the obvious response in these cases (and also for those who believe Jesus didn't literally pre-exist) is that 'all' has to be scoped to a context. When Paul says 'all have sinned', he means to exclude Jesus who he takes was fully man, just as when Paul says 'all has been created through him' (Col. 1:16) he is obviously exempting the Father and the Son himself (and, if you think Paul is referring to the new creation, the 'all' is meant to apply rather to everything pertaining to the new creation - "The Kingdom achieved new pinnacles in science and technology - all learning came about because of laws put in place by the King" obviously refers to learning within the Kingdom).

So the simple answer is the Bible only teaches that all men are sinners if you read it woodenly. Rather, the Bible teaches Jesus was fully man but unique in being sinless.

The next question, for anyone who holds Jesus is fully man (JWs, but also Trinitarians or Unitarians) is why he was able to resist sin when other humans didn't? Is there something unique about Jesus?

The Trinitarian answer is that Jesus is also fully God, and that is what enabled (or perhaps necessitated) a sinless life despite being fully man.

The JW answer is that Jesus as man was conceived uniquely, directly by God, and so did not receive the tendency toward sin that those who have human fathers have.

As the JW article Did the imperfection of the virgin girl Mary have a negative effect on the conception of Jesus? says

"Could a child conceived in this way by an imperfect woman be perfect and free from sin in his physical organism? How do the laws of heredity work when there is a union of perfection with imperfection? Remember that holy spirit was employed in transferring the perfect life force of God’s Son and causing the conception. This canceled out any imperfection existing in Mary’s ovum, thereby producing a genetic pattern that was perfect from its start."

So JWs hold that Jesus was uniquely able to be sinless because he was uniquely conceived by the HS, thus avoiding the consequences of Adam's sin.

From their own literature, these points are made:

"At Isaiah 9:6 Jesus Christ is prophetically called 'El Gib.bohr', "Mighty God"...

The Bible refers to Satan the Devil as 'the god of this system of things.' (2Co 4:4) That Satan is the 'god' there referred to is clearly indicated later in verse 4 where it says that this god 'has blinded the minds of the unbelievers.' At Revelation 12:9 he is said to be 'misleading the entire inhabited earth.' Satan's control over the present system of things, including its governments, was indicated when he offered Jesus 'all the kingdoms of the world' in exchange for 'an act of worship.' - Mt 4:8,9." Insight on the Scriptures, Vol. 1, pp. 968 & 972

This contrast between the mighty godship of Satan and the 'Mighty God' designation of the coming of Christ as Messiah is necessary, in order to show that there are, indeed, different nuances of meaning in the biblical use of the words God/god. Of note is the appearance of Satan, when Jesus was on earth as "the perfect example of conduct manifesting godly devotion. - 1Ti 3:15" (Ibid. p. 972) All Jehovah's Witnesses would say Satan is a false god, and that Jesus being 'a god' does not attach any falsehood to him. Significantly, that quote about Satan indicates the false god having great might and authority on earth, having kingdoms to offer the man, Jesus, the Mighty God of Isaiah 9:6. This is where consulting Volume 2 on the person of Jesus Christ, when on earth, is necessary.

On p. 54 Isaiah 9:6 is quoted again after it has been established that there is only one true God and, as the Word of God, who became flesh as Jesus, is said to have been created at a certain point in time by the one true God, this explanation is offered about how Satan compares with Jesus with regard to godship:

"These facts give solid support to a translation such as 'the Word was a god' at John 1:1 [the translation given in their New World Translation]. The Word's preeminent position among God's creatures as the Firstborn, the one through whom God created all things, and as God's Spokesman, gives real basis for his being called 'a god' or mighty one. The Messianic prophecy at Isaiah 9:6 foretold that he would be called 'Mighty God,' though not the Almighty God, and that he would be the 'Eternal Father' of all those privileged to live as his subjects. The zeal of his own Father, 'Jehovah of armies,' would accomplish this. (Isa 9:7) Certainly if God's Adversary, Satan the Devil, is called a 'god' (2Co 4:4) because of his dominance over men and demons, then with far greater reason and propriety is God's firstborn Son called 'a god,' 'the only-begotten god' as the most reliable manuscripts of John 1:18 call him." Insight on the Scriptures Vol. 2 p 54

The line of reasoning presented is that the Archangel, Michael, was created as the first of Jehovah's creation, this mighty angel also being called the Word of God, who later became the man, Jesus.

“…it appears that Jehovah God caused an ovum, or egg cell, in Mary’s womb to become fertile, accomplishing this by the transferal of the life of his firstborn Son from the spirit realm to earth. (Ga 4:4) Only in this way could the child eventually born have retained identity as the same person who had resided in heaven as the Word…

Mary was a descendant of the sinner Adam, hence herself imperfect and sinful. The question is therefore raised as to how Jesus, Mary’s ‘firstborn’ (Lu 2:7), could be perfect and free from sin in his physical organism. While modern geneticists have learned much about laws of heredity and about dominant and recessive characteristics, they had had no experience in learning the results of uniting perfection with imperfection, as was the case with Jesus’ conception. From the results revealed in the Bible, it would appear that the perfect male life-force (causing the conception) canceled out any imperfection existent in Mary’s ovum, thereby producing a genetic pattern (and embryonic development) that was perfect from its start. Whatever the case, the operation of God’s holy spirit [‘the perfect male life-force’?] at the time guaranteed the success of God’s purpose. As the angel Gabriel explained to Mary, ‘power of the Most High’ overshadowed her so that what was born was holy, God’s Son. God’s holy spirit formed, as it were, a protective wall so that no imperfection or hurtful force could damage, or blemish, the developing embryo, from conception on.-Lu 1:35” (Ibid. p56)

The Word was used to create everything else, including another mighty angel who later became Jehovah's great Adversary. This great Adversary was the Satan who tried to tempt Jesus by offering him all the kingdoms on earth. The latter was a false god, but Jesus was a faithful mighty one (a god) who although created himself, was the one in his pre-human existence who created the mighty angel who became Satan. It's an interesting theory. But, of course, it depends utterly on the Word / Michael / Jesus being a creature, just like Satan is a creature - created. In that sense, both Jesus and Satan can have 'a god' designation applied to them as both of them are mighty ones, with the Jehovah's Witnesses differentiating their Jesus as never having sinned, while Satan did. A related link on how they view Jesus being called "God" in John 20:28 is at How do Jehovah's Witnesses explain Thomas's exclamation, "My Lord and my God"?

The Jehovah's Witness explanation also invokes their translation of John 1:18 ("the only-begotten god"). That particular verse has already been examined in detail on the Stack Biblical Hermeneutics site, looking at particular manuscripts, so that would be a good place to go to next. https://hermeneutics.stackexchange.com/questions/69939/uniquely-divine-begotten-born-one https://hermeneutics.stackexchange.com/questions/6/should-john-118-read-the-only-begotten-god/79535#79535