Muhammad

First off, I would like to give credit to Karen Armstrong, whose writing is incredibly informative, but at the same time light and engaging. A Prophet for our Time reads more like a novel, but has the educational depth of a textbook.
The introduction and first chapter discuss Muhammad as an individual before he becomes the prophet, detailing what was his childhood and moving along until his experience on the mountain. Though he lost his parents, it seems as though Muhammad lived a relatively charmed life for the time, having individuals watch over him for a great portion of it, starting with his grandfather. I've yet to formulate an opinion on the information of the book, or find any glaring questions that it leaves me with, so I'll reiterate what we talked about in class, in the hopes that we can discuss it a little further here.
I was under the impression that Christianity and Judaism, though recognized as at least followings of god, were simply tolerated by the Muslims, and that Muslims thought them inferior or misguided in several ways. According to Armstrong, however, Muslims considered Christianity and Judaism as part of Islam, and that they were all essentially one and the same. In class we discussed how loyalty to one's tribe came above everything else, and that Muslims and, in this case, Arabs would do anything for their tribe. This meant giving one's life for it and ensuring that one never fights with his tribesmen. This is part of why Islam was so influential; it outlawed any fighting within the tribe if Islam. This supposedly worked while Muhammad was still alive, but when he died, this ideal began to die as well. After his death, the tribes began to expand the empire into Christian-held territories. My argument, or query, then, is centered around the idea that the Muslims that began to raid and kill Christians. It is no secret that there were several wars in which both Christians and Muslims were aggressors. Wouldn't this go against the beliefs of the Muslims to not kill any of their tribesmen, if Christianity was seen as a part of that?
Professor Silliman suggested that this all occurred after Muhammad's death, and he had no say over it. Silliman also proposed the idea that Muhammad was far too busy ensuring that his people weren't slaughtering themselves and creating a religion to really worry about Christianity and Judaism. As well, there were inherent necessities that Muslims had, and the only way they could obtain and fulfill those was to pillage. I understand these points completely, but I'm skeptical about the willingness of his followers to do so. Were they so far removed from Muhammad so soon after his death that they didn't realize they were going directly against his (and thus God's) word? Did they simply not care? This is a culture that has scholarly skepticism built into its very foundation, and yet the people subscribing to the religion didn't understand this violation of holy law?
I asked a lot of questions here, and I don't expect answers from all of them. I think, also, that professor Silliman did a very good job of defending their actions as necessary, and at this point we can only speculate on what really went on, but I am left with lingering doubt about the inconsistency of the religion and the actions of its people.