Not True Muslims

It's a funny thing, this argument I hear with some regularity that so-and-so was not a true Muslim because no true Muslim would have done such a thing. I gather a true Muslim is virtuous, and since the act wasn't virtuous, the person's self-identification as a member of the community was false. Of course it must be the case that no truly virtuous person commits vicious acts. But surely this is all beside the point, which is that sometimes religious beliefs play a part in some vicious acts. I'd be willing to say that not all religious beliefs are true, and that the false ones can sometimes be hazardous to oneself and others. 

Not all religious beliefs can be true,

Since not all religions agree. 

This may seem controversial to you,

But it seems like sound logic to me.