The things you describe are permutations of how any political organization behaves. For clarity, the class of political organizations that use force/violence. That class includes all governments.
Filming ritualistic beheadings is a propaganda technique. It helps both ISIS and the US. This is why the filmed beheadings are played for the US audience by American media. The meaning of the beheadings is different for both audiences, but ISIS continues to produce the videos because it views the tradeoff as being beneficial to its growth and survival.
Rapes are unfortunately a standard part of military conduct. Due to financial constraints, combatants must be recruited with promises of glory in the afterlife, but the more secular among them may join just for the opportunity to have sex. This is sad and extremely ugly, but it is a response to resource scarcity and the need to recruit fighters. The constituents must consent to military action by joining, and the compensation (supernatural or earthly) is the price of that consent.
In the US, soldiers are recruited from the ghettos, slums, and backwoods of America. Many are desperate to be educated and to become employable because the K-12 system has failed them. In the first world we can pay our soldiers with salary, training, and subsidized loans. This is not possible without significant wealth, which is why a professional class of soldiers did not exist in Europe until economic development made it feasible.
Suicide bombings are not any different from drone strikes. For a given drone strike, there is some nonzero probability of civilian deaths. The bar is pretty low, as we've found out. You can call them "accidental" but that is a propaganda word. The truth is, the value we attribute to a human life in regions where we have drone strikes is appallingly low.
Further, the purpose of suicide bombings is terror, which is the same purpose that drone strikes serve. The potential victims are intended to lie awake at night hearing the buzz of propellers, unable to sleep soundly in case a bomb rains down upon their home. The intention of flying lots of drones everywhere is to make the US force projection seem unstoppable and ubiquitous, so that the locals get fed up with living in terror of being "accidentally" killed and stand up to their leaders.
This is the same goal of suicide bombings, incidentally.
Rhetoric about entire populations of people and all the religious rhetoric is part of ISIS' growth strategy. For historical reasons, secularism is associated with occupation and mistrust, and Islam is largely decentralized (friendly to pluralism). It makes sense to attach any grassroots movement to the existing social structures and network of Islam. This both creates a vocabulary for the struggle but also a moral code that has stood the test of time. This happens everywhere... Religion was used in the American abolitionist movement and anti-slavery movement, as well as the civil rights movement. It is used in America today to support the war on terror as well. ISIS is simply doing the same thing. Over time, if ISIS gains power it will moderate its stated views in order to appeal to a wider audience. Early adopters like highly opinionated, absolutist rhetoric and ISIS knows that. It can be safely ignored as irrelevant to ISIS's slow process of institution building, and can also be considered irrelevant to ISIS's military strategy.
> if your politics takes violence to convince others, you are not a political movement, but a terrorist organization
Was this true of the colonists in the American revolution? How do you determine the legitimate use of lethal force by a political organization. I'm curious how you reason about this. My take is that it's all equally illegitimate, but strongly desired by all political organizations (power aggregators). Power is useful because it lets the group control some aspect of society. Without the need to control, there would be no need for power and no need to consolidate power.
> yet the libertarian grassroots movement doesnt resort to violence of any kind to further its political agenda. same can be said for the green party or tea party.
This is true, but not for the reason you suppose, but because in the US there is already a government monopoly on force. This is why the issue of gun control is so clear-cut for most people. Clearly nobody is going to form a successful militia with their 2nd amendment rights, so why worry about that aspect of the 2nd amendment. Gun control is considered actionable precisely because guns could be easily taken by force from all gun owners if the government desired to do so.
The goal of all political organizations is to a) take power, and b) control a monopoly on lethal force in their region of influence. Whether we like it or not, and whether we agree with it or not, this is a law of the universe of politics. It's an artificial distinction to claim that simply because organizations in the US do not seek to overturn the government's monopoly on force that a monopoly on lethal force isn't the first and foremost priority of every government.
The Green and Tea party are not revolutionary movements. If one of them started to look like one, it would quickly be categorized as a terrorist organization. Consider how quickly the Patriot Act allowed officials to classify all sort of behavior as "terrorism". There is no room in the US for any group to attempt this sort of thing.
I do not think of this as good or bad, it's just the way human political groups work.
> To identify what is morally wrong, you need to determine the reasoning behind the action.
This is true, but I think that your view of ISIS comes from some convenient definitions about what constitutes war and what constitutes a combatant:
Definitions I'd use: A war is a political dispute that has become violent. The notion of a combatant came out of Geneva Convention-era conflict involving major powers. In an asymmetrical conflict such as the US vs ISIS, the notion of a combatant is of limited usefulness.
This is why the US sends captives to Gitmo, because an offshore location was needed to get around the legal ambiguity of doing so.
ISIS (or any small group aka "terrorist" organization) will attempt to break the enemy's will in any way possible. This includes attacking civilians, because there are few American combatants in the region, and traditional military style campaigns against American forces would be doomed.
This brings us to the goal of war. Why fight a war? To attempt to break the enemy's will to continue fighting. Historically, the US has been flexible about what it considers acceptable conduct depending on the probability of victory. The lower the probability, the more slack is allowed in the definition.
So I'd argue that if you are ISIS you will do as many acts to break the will of Americans as you possibly can, and you won't worry about it if some civilians get killed. Surely you'd rather kill military members, but if doing so doesn't seem possible you'll target civilians. Terrorism is a tactic by which an attack creates fear disproportional to the actual amount of damage (property damage and loss of life).
So unless you think ISIS should just give up, I think it's hard to find fault with its tactics on a moral basis. Yes, I disapprove of the tactics, but I also disapprove of using drones the way the US does, and the many other things the US does that are essentially terrorism.
More broadly, political conflict will always exist between nations. I'd prefer it if my own country was completely transparent about the objective of any use of force. National interest is fine, but we are told that the US is destroying the middle east for noble, moral reasons. This is far from true.