Hina Azam has written an article called Terrorism: A Return To Jahiliyya (Jahiliyya is an Arabic word meaning "pre-Islamic period of ignorance").
This is something I've often thought myself and said in other contexts: these people are acting like Islam had never come to them.
Some highlights:
The truth of the matter, however, is that they are engaged in the very behavior that the Qur'an and Prophet came to combat: tribalism. Despite protestations to the contrary, Al-Qaeda and similarly-minded groups are engaged in no more than the old-fashioned tribal warfare, the hallmark of jahiliyya... ...But it is not the political nature of their motivation in itself that is illegitimate from an Islamic perspective. The illegitimacy lies in their methods. In the discourse of the terrorists, religious texts are being twisted in order to support the pre-Islamic practice of vendetta (tha'r) - the very approach to socio-political conflict that the Qur'an and the Prophet outlawed. Pre-Islamic Arabia was a society in which there was no central authoritative body to oversee justice or to mete out punishment for injustice. It was a society in which the only commonly-recognized law was the law of tribal vengeance: If someone from tribe A attacked or harmed someone from tribe B, the attack was taken as license by anyone in tribe B to retaliate against anyone and everyone from tribe A. The tragic result was bloodshed that would touch a far wider circle than the original assailant. The Qur'an sought to put an end to this murder and mayhem through a series of moral and legal principles dictating how human beings should live with one another, both inter-tribally and intra-tribally. No longer was it legitimate for anyone from tribe B to kill anyone from tribe A, no matter how great the desire for vengeance. In the civilian realm, this is the principle behind the law of qisas: the only one who could be prosecuted was the one who has committed the crime, whether it be murder or injury. At the level of the state, the Qur'an laid down principles governing warfare, principles that the Prophet and the scholars interpreted as delineating fundamental laws -- such as the distinction between combatants and non-combatants, and the illegitimacy of attacking the latter.