Belief in religion necessitates faulty reasoning. Religious belief, Using the standard definition that involves the supernatural, Necessarily requires erroneous, Illogical thinking. Every claim for a deity involves a logical fallacy taking place. Perhaps a deity exists, But it is illogical to think so. Even if God came down from the heavens to tell me he is real, It would be more probable that I was delusional or hallucinating, Or that I am actually int he matrix, Or someone or something is playing a trick on me, Etc. Etc.
There is no evidence. Is a devine being possible? Maybe. Can we know with 100% certainty whether or not one exists? No. But we have rational thought and probability, And those are how you answer the question. The facts are there is zero legitimate pieces of evidence for divinity, You necessarily must commit logical fallacies in order to believe, The burden of proof is one the one claiming there to be divinity, And it is an extraordinary claim, Which requires extraordinary evidence. Replace divine being with the Easter Bunny. We can't say for certain, But the probability is so low that it is appropriate to say that the Easter Bunny does not exist.
No true Scotsman fallacy to say they aren't "real" Muslims. Much of religion is interpretation. Even a true religion of peace, Like Jainism, Could have its teachings be warped and its followers start killing. They would still be jain, Despite the mental gymnastics they undertook to come to their conclusion.
As for this particular question, One doesn't even need to go that far. The pilots had a very reasonable, Honest interpretation of the Quran and hadiths. Muslims can say things like the Quran teaches that killing is wrong, But there are many verses in it that promotes killing non-Muslims and even where it says killing is wrong it says afterwards that it's not wrong if there is corruption involved. And, Of course, The Quran teaches that corruption includes apostasy, Being a non-believer, Homosexual, Against Islam, Etc. Etc.