I have ever had a discussion with a Christian about ISIS. He claimed Islam (Quran) makes people become warrior. He claimed that Quran makes people become evil of the world. I explained him that Quran is not a simple book. Many people went astray as they read and comprehended Quran without any guidance by the scholar. They didn't understand it deeply. They were not great thinker.
He replayed, "So, it's true that Quran makes people become warrior and killing others?"
I replayed that it was only one bad effect. In Sunni Muslim tradition, we understand Islam through Quran and Hadith, and the explanations of Companion Scholars, Tabi'een Scholars, Taba' Tabi'een Scholars, Third Century Scholars, and so on. Why our understanding should be proven by them? It's about reading comprehension.
There are some level of reading comprehension of people. Our professor at university often said, "Support your statement with scientists statement." Yes, if we talk about something which is not trivial or sacred we may not do it without any much consideration.
Therefore, if anyone just read Quran, they will not understand Islam. Something makes human different from animal is that human can think. Yes, human can think, while animal can't.
He asked e again, "Why God created such unclear / illlogical book?"
As I know, in my religion, it's not permissible to question or investigate God. But, I replayed with my understanding about logic. If everything is clear, do all human will live their life with passion? But, I don't said Quran is not clear. Qurann is so clear for the believer. Another thing I want to say is that in this life most everything is in pair: Dark and bright, good and bad, logical and illogical. We can say something is tall because we have seen other things are short. So, clear or unclear is not a problem. If everything God does is the same as what human does, so God is the same as human. If God is the same as human, what for we worship to him?! He is beyond human's understanding.
In Quran, God orders all Muslim to keep learning / studying / searching knowledge until they die. It make sense, human should never stop learning because life is always change.
After all, I said to that Christian man, "We should respect every religion's subjectivity." And he agreed with me. Every individual will claim themselves the rightest one, so does every community.
"All living things must be exist together in harmony," Albert Einstein at the Young Einstein Movie.
You may like to read How Umar Ra. (The Prophet's Companion) treated Non-Muslims