What Led Some Of The Ahlul Kitaab [some of the Jews and Christians] to Behave In Such An Unpleasant Manner?!
Allaah (The Most High) said:
وَمِنۡ أَهۡلِ ٱلۡكِتَـٰبِ مَنۡ إِن تَأۡمَنۡهُ بِقِنطَارٍ۬ يُؤَدِّهِۦۤ إِلَيۡكَ وَمِنۡهُم مَّنۡ إِن تَأۡمَنۡهُ بِدِينَارٍ۬ لَّا يُؤَدِّهِۦۤ إِلَيۡكَ إِلَّا مَا دُمۡتَ عَلَيۡهِ قَآٮِٕمً۬اۗ ذَٲلِكَ بِأَنَّهُمۡ قَالُواْ لَيۡسَ عَلَيۡنَا فِى ٱلۡأُمِّيِّـۧنَ سَبِيلٌ۬ وَيَقُولُونَ عَلَى ٱللَّهِ ٱلۡكَذِبَ وَهُمۡ يَعۡلَمُونَ
‘’Among the people of the Scripture (Jews and Christians) is he who, if entrusted with a Cantar (a great amount of wealth, etc.), will readily pay it back; and among them there is he who, if entrusted with a single silver coin, will not repay it unless you constantly stand demanding, because they say: “There is no blame on us to betray and take the properties of the illiterates (Arabs).” But they tell a lie against Allah while they know it.’’ [3:75]
[a] They made corrupt claims and held repulsive views, which made them see themselves as possessors of great status, whilst the illiterate Arabs are extremely degraded. Therefore, the rights of the illiterate Arabs could be violated. So by way of these corrupt claims and repulsive views they ate haraam and considered it lawful by way of creed.
[b] Likewise, they lied against Allaah because the scholar who declares things lawful or unlawful is considered- by the people- as one transmitting a ruling of Allaah and not a ruling from himself; and that (ruling of theirs about the wealth of the illiterate Arabs) is a lie. Allaah said: [وَيَقُولُونَ عَلَى ٱللَّهِ ٱلۡكَذِبَ وَهُمۡ يَعۡلَمُون] ‘’ But they tell a lie against Allah while they know it.’’ This is from the greatest of sins—speaking about Allaah without knowledge.
[Source: Taken from Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan of Imaam Sadi (rahimahullaah)]