As long as there is one exception to the rule, then the rule must be false?
That may be true at some level, but for everyday living, it's better to go with the percentages. If ten percent of the muslims are terrorists, then you should be ten percent more careful around muslims.
At any one time, how many guns are loaded? I'd venture a wild guess that 80% of ALL guns are unloaded. But, hold any random gun, you must always presume it to be loaded.
Same for muslims... ALWAYS assume the muslim is loaded. It just might be.