If Gabriel really did visit Mohammed, he possibly didn't listen very carefully, or we don't have the true account of what was said, or Mohammed was overcome by the Adversary, which always attacks those whom the Lord has chosen to receive special revelation. Otherwise, it wasn't a Heavenly messenger at all, and possibly worst of all, was an actual message from the Pretender himself. In any case, Islam quickly fell into the same embroiled Apostasy, in which Judaism and Christianity were already well involved. Satan always takes advantage of our imperfections and shortcomings, especially of those with gumption and/or those who are in a position to have done the most good with it, if they don't completely fail, and succumb to the effects of their own weaknesses. Which is not to say that having weaknesses is what makes us fail, just giving in to the effect of having those weaknesses used against us, whether it be by the world, by nature, or even directly by impostors, posing as man or spirit. They have great symmetry, and want us to be miserable like them, and always have a counterfeit version of the Truth. They too demand sacrifices, humiliation, holocausts and broken heart. But for their own glory. Alone.