You are asking a legal question–i.e., is the murder of civilians for political purposes sanctioned by Islamic law? The Quran has very little legal content in it, and like the entirety of the text the passages which discuss warfare have always been read alongside companion historical works which situate the relevant verses in their context. So the Quran is not the only source when it comes to legal precedent; it rarely features in legalistic texts as a primary source of theoretical and applied law. So we turn instead to the most mainstream and widely-read legal scholars.

The answer as to the permissibility of terrorism is a resounding no. In Islamic just war theory, it is explicitly forbidden to harm civilians (read: non-soldiery) or destroy civilian infrastructure. Jihad is properly a defensive warfare in the legal context–it is to be undertaken when Muslims outside the Islamic imperium are being threatened and/or their rights curtailed. You are also liable to be killed if you convert away from Islam or prevent Muslims from exercising their religion, but in the historical archive very few people were ever killed only for being apostates.

These are largely medieval precepts. In the modern context, where nation-states on the secular western model have become the prevailing political structure in the Islamic world, the only legitimate warfare that can be undertaken is by the nation-states themselves, who alone possess a monopoly on legitimate violence. Jihad, if you can call it that, occurs when these states frame national warfare in religio-cultural terms–defense of fatherland, of Muslims whose religious identity has been coopted and conflated with their national identity. You will find national actors sometimes referring to Quranic passages or legal precedents very loosely and unsystematically as a means of further legitimating the use of violence. You will also see it defined as a crime in these states sometimes to convert away from Islam; again, this has to do with the above, where religious identity is conflated with national identity, and forsaking it is akin to committing treason against the nation. This is why there are many more examples of apostates being put to death in the modern period, when the nation-state was adopted as a political unit in the colonial-era Middle East with the encouragement of European powers.

The "jihad" which occurs independently of this by terrorist cells is also framed as such for the same legitimating reasons.