The Quraish broke the treaty, not the Bani Qurayza.

The Banu Qurayza (Arabic بني قريظة; بنو قريظة alternate spellings include Quraiza, Qurayzah, Quraytha, and the archaic Koreiza) were a Jewish tribe who lived in northern Arabia during the 7th century, at the oasis of Yathrib (now known as Medina). In 627 CE, the tribe was besieged by the Muslims commanded by Muhammad, who charged the tribe with treachery for not aiding the Muslims during the Battle of the Trench. The Qurayza were taken captive and all men, apart from a few who converted to Islam, were beheaded, while all women were enslaved.[1][2]
http://en.wikipedia.org/wiki/Banu_Qurayza

Ibn Ishaq recorded that after Muhammad arrived in Medina in 622, the Arabs and Jews of the area signed an agreement, the Constitution of Medina, which committed the Jewish and Muslim tribes to mutual cooperation. The nature of this document as recorded by Ibn Ishaq and transmitted by ibn Hisham is the subject of dispute among modern historians many of whom maintain that this "treaty" is possibly a collage of agreements, oral rather than written, of different dates, and that it is not clear when they were made or with whom.[14][15][2] Watt holds that the Qurayza and Nadir were probably mentioned in an earlier version of the Constitution requiring the parties not to support an enemy against each other.[2]

Aside from the general agreements, Muslim sources including the chronicles by Ibn Ishaq and al-Waqidi, contain a report that after arriving to Medina, Muhammad signed a special treaty with the Qurayza chief Ka'b ibn Asad. Ibn Ishaq does not name his sources for this claim; al-Waqidi mentions two sources: Ka’b ibn Malik of Salima, a clan hostile to the Jews, and Mummad ibn Ka’b, the son of a Qurayza boy, who was sold into slavery after the massacre of the Qurayza men and subsequently became a Muslim. According to Watt, both sources may be biased against the Qurayza, and on these grounds the historicity of this agreement between Muhammad and the Banu Qurayza is open to grave doubt.[2] Norman Stillman furthermore argued that the Muslim historians had invented this agreement in order to justify the later massacre of the Qurayza men and the enslavement of their women and children.[16] On the other hand, R. B. Serjeant is more optimistic about this agreement and infers that Banu Qurayza knew the consequences of treachery.[17][verification needed]
http://www.answers.com/topic/banu-qurayza

005.116 And when Allah saith: O Jesus, son of Mary! Didst thou say unto mankind: Take me and my mother for two gods beside Allah ? he saith: Be glorified! It was not mine to utter that to which I had no right. If I used to say it, then Thou knewest it. Thou knowest what is in my mind, and I know not what is in Thy Mind. Lo! Thou, only Thou, art the Knower of Things Hidden ?

Luke 10:1-16
2 And He was saying to them, "The harvest is plentiful, but the laborers are few; therefore beseech the Lord of the harvest to send out laborers into His harvest. 3 "Go your ways; behold, I send you out as lambs in the midst of wolves. 4 "Carry no purse, no bag, no shoes; and greet no one on the way. 5 "And whatever house you enter, first say, 'Peace be to this house.' 6 "And if a man of peace is there, your peace will rest upon him; but if not, it will return to you. 7 "And stay in that house, eating and drinking what they give you; for the laborer is worthy of his wages. Do not keep moving from house to house. 8 "And whatever city you enter, and they receive you, eat what is set before you; 9 and heal those in it who are sick, and say to them,' The kingdom of God has come near to you.' 10 "But whatever city you enter and they do not receive you, go out into its streets and say, 11'Even the dust of your city which clings to our feet, we wipe off in protest against you; yet be sure of this, that the kingdom of God has come near.' 12 "I say to you, it will be more tolerable in that day for Sodom, than for that city. 13 "Woe to you, Chorazin! Woe to you, Bethsaida! For if the miracles had been performed in Tyre and Sidon which occurred in you, they would have repented long ago, sitting in sackcloth and ashes. 14 "But it will be more tolerable for Tyre and Sidon in the judgment, than for you. 15 "And you, Capernaum, will not be exalted to heaven, will you? You will be brought down to Hades! 16 "The one who listens to you listens to Me, and the one who rejects you rejects Me; and he who rejects Me rejects the One who sent Me."
NAS