If only. There might then be some hope of a reformation.
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Strickly forbidden by Mohammad. Potential reformers have an automatic fatwa of death placed on them - every muslim is obligated to kill them. (Reformers you see on TV are practicing taqyyia - sanctioned lying in defense of the movement to deceive infidels).
Christianity can be reformed - Islam cannot. There are many variations of the Bible, but only one version of the koran.
Christ killed no one. Mohammad killed 400 by his own hand - all Jews (beheading, a technique he popularized).