a lot of things have been said about muslims that either aren't true, or not entirely.
keep in mind that their system holds that infidels aren't human. If you aren't a muslim, then you may as well be a goat or a sheep. You may be lied to, cheated, stolen from, tortured, raped, etc, as you simply don't count.
the more extremist among them have taken this to the point where they justify other muslims as not being "real" or "good enough" muslims, so they have an excuse to kill them, too.
The bottom line is that the majority of muslims on this planet aren't really any problem for us. But clearly some of them are. Identifying them is the problem to be solved.