"Thou shalt not kill" is part of the ten commandments of the Old Testament, which is certainly not pacifist. It's not really at the core of the message of Jesus as we can attempt to reconstruct it - instead of a list of concrete rules, he seems to have been more inclined toward distilling the Law into general principles; Mark 12:38-41:

And one of the scribes came up and heard them disputing with one another, and seeing that he answered them well, asked him, “Which commandment is the most important of all?” Jesus answered, “The most important is, ‘Hear, O Israel: The Lord our God, the Lord is one. And you shall love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.’ The second is this: ‘You shall love your neighbor as yourself.’ There is no other commandment greater than these.”

Jesus is contrasted with the Rabbinical Pharisees, who constructed elaborate schemes to avoid accidentally even coming close to breaking any of the laws of Moses - you could perhaps losely compare them to the "Ultra-Orthodox" Jews of today, or (quietist) Salafi Muslims.

Was Jesus a pacifist? I think the most we can say with relative certainty is that he was an apocalypticist - he believed there would soon come a day of reckoning and that the Son of Man - a cosmic judge - would arrive, when the wicked would be punished, the Romans overthrown, and a paradise on Earth established. In Matthew 19:27-30, Jesus has just talked about entering the kingdom of God:

Peter answered him, "We have left everything to follow you! What then will there be for us?"

Jesus said to them, "Truly I tell you, at the renewal of all things, when the Son of Man sits on his glorious throne, you who have followed me will also sit on twelve thrones, judging the twelve tribes of Israel. And everyone who has left houses or brothers or sisters or father or mother or wife or children or fields for my sake will receive a hundred times as much and will inherit eternal life. But many who are first will be last, and many who are last will be first."

Jesus is suggesting an inversion of privilege and power here; his humble disciples will be princely - those who are wealthy and powerful now will, perhaps, be servants. Despite this passage only appearing in Matthew, there's some reason to believe it is an early tradition, even something Jesus himself said - among the twelve is Judas Iscariot, and it would seem strange to invent a tradition where Judas Iscariot gets to sit on a throne and judge one of the twelve tribes. Moreover, it would actually provide a basis for explaining the charge of Jesus calling himself, "the king of the Jews", something he never does but refuses to deny, in an exchange that doesn't mesh too well with the preceding confrontation by the Sanhendrin, in any of the gospels. It is even possible that Judas Iscariot's betrayal was not providing the Romans with Jesus' location (which seems unlikely - they could just have followed him!), but informing them of what he had said to his disciples.

We can argue about other passages (e.g. "do not think I come to bring peace, I come to bring a sword..."), but that teachings such as "turning the other cheek", "loving thine enemy" or "rendering unto Caesar" suggest nonviolence is hard to deny. The best explanation of these teachings in the context of apocalypticism I've come across comes from Bart Ehrman - Jesus believed that in the kingdom established by the Son of Man, there would be no need for violence, hatred or rebellion, and therefore, the righteous should prepare and show their conviction in the coming of this kingdom by acting as if it was already here.

In Jesus' lifetime, then, his teachings could well have appealed to the poor, oppressed and powerless - especially, perhaps, those who felt unable to follow the strict teachings of the Pharisees. Remember that Jesus would not have necessarily needed a large following in his own lifetime - 20-40 close followers would have been enough. In fact, given that they appear to have been convinced that he had risen from the dead based on a vision experienced by his brother James (according to Paul) a small, tight-knit following seems very likely indeed. That doesn't mean Jesus could not have attracted significant crowds in Jerusalem - by all accounts, he must have been an extremely charismatic and compelling man, but these would of course not all have immediately joined his inner circle.

The idea of a message centered on the plight of the oppressed and the evils of violence was hardly new, either. About 1300 years before Jesus, these ideas were at the core of the message preached by Zarahtustra, an Iranian cleric from a pastoral society in Central Asia. His message centered on the suffering of herders whose cattle were stolen, the cries of the mystical "ox-soul" at this disruption of nature, and the impotence of the oppressed. He lived in an age where society was changing owing to the proliferation of bronze age warfare - chariot raids and plundering, and where, as we see attested in the Rgveda, cults formed around martial deities like the strongman Indra, celebrating warfare. Zarathustra condemned these Daeva as entities that could not distinguish Good from Evil. In the surviving seventeen great hymns, the Gathas, we find a still-compelling claustrophobic narrative structure wherein Zarathustra pleads to the almighty Mazda, the personification of wisdom, expressing doubts about whether the deity truly possessed the power to destroy evil. Zarathustra concludes eventually that there would be the coming of an age of reckoning in which "those who bring benefit" empowered by devotion, power and righteousness would judge evildoers, who would suffer divine punishment. Eventually, good would be separated from evil, and all men would enjoy eternal life. Liturgical bits like the "Zoroastrian pledge" of Yasna 12 contains promises like "I will never again raid a Mazda-worshipping settlement" and a pledge to follow "the Mazda-worshipping thought that puts down the weapon and ceases the attack".

These teachings (which are found more developed in hymns composed around 1200-500 BC) most likely laid the seed for what would over hundreds of years of development, cultural exchange and recontextualization become the apocalypticism we find in Second Temple Judaism, early Christianity, early Islam, and explicitly attested in medieval Persian Zoroastrian texts.