Islam must be Judged by its sublime principle not by the bad conduct of some muslims

Bismillahirrahmanirrahim,

If you find a doctor who adopts harmful medical procedures or a teacher with bad moral character, you will certainly disapprove of their wrong practices, which are obviously at odds with their social position and the type of knowledge they have acquired. This, however, will not make you change your mind about the great benefits medical science has afforded mankind or great position education and learning occupies in society and civilisation.

You will undoubtedly reach the conclusion that such a doctor or teacher actually misrepresents his professional qualifications and affiliations. 

By the same token, if you find some muslims who follow some bad practices, you may mistakenly assume that such practices reflect the spirit of islam, which is obviously not true. Just because the wrong practices of that wrong doctor or teacher cannot be possibly attributted to the medical or educational profession, such muslims' bad practices cannot, with an even stronger reason, be attributed Islam; They merely constitute an aspect of human weakness and could therefore be attributed to wrong cultural practices which have nothing to do with islam.

Source: The New Muslim Guide (Book)