I know how Muhammed started. I wish he would have stayed that way - a peaceful, tolerant merchant who wanted to reform local religion. At first he never even said there was only one god he just emphasized the primacy of the one god he was talking about over other ones. I don't think ethics so much as political advantages attracted people to Islam, at least not after he became a warlord. It's like Christianity in Europe, the church made the benefits of joining (ie a bureaucracy to collect taxes, write law codes, etc) clear to barbarian rulers who then converted for political rather than ethical reasons.
Like I said I'm not bashing the church in the middle ages, and I certainly don't think that warring barbarians would have been better; I'm not an anarchist and I think that you need a state of some kind (on the political spectrum I ended up being about -6 and -6), and hell even if you don't want one you're going to get one, the question is just of what kind. Power vacuums resolve themselves, the question is just how nasty the resulting social hierarchy is.
Re: ingroup/outgroup. Of course cultures need to negotiate. Still, too much negotiation leads to the west bartering away its own values in exchange for... what? Sharia is one example. Many western citizens are so (correctly) sorry for the imperialistic excesses of the past that they are willing to sanction almost anything in the name of tolerance (for example,
http://standpointmag.co.uk/node/2042/full). In my view, culture stops where human rights begins. I can and do appreciate the literature, philosophy, history, architecture, art, legends, technologies, etc, of other cultures, but I think things like free speech, women's rights, democracy, etc, are up for negotiation about 0 %. There's such a condescending double standard going on too. When fundamentalist Mormons practice polygamy and underage marriage, the cops raid them. When fundamentalist Muslims do the same, suddenly the attitude is "it's just their culture" as if "well they aren't from the west, so they can't be expected to know any better." That kind of "tolerance" is condescending.