Threatened with a knife by an ISIS member!

This story happened three years ago when I decided to move from Dubai (UAE), one of the safest countries in the world, to Turkey wanting to pursue my bachelor's degree in energy engineering. Oh and I do have to mention that I'm from Syria but spent only half of my childhood there. Anyways, It was my first day in Istanbul. My brother was showing me the city. I went with him to Taksim square holding my guitar because I wanted to fix it. A guy stopped us and said with a Syrian dialect "It's a shame to see the Syrian youth playing guitar when people are dying in Syria". " What is your problem?" My brother replied.
"Why are you not in Syria fighting for justice?"
"Why don't YOU go and fight in Syria?!" The guy lifted his shirt and showed his injuries and told us that he's in Istanbul for recovery and he also showed us an Austrian ID or an EU card that says he's an Austrian citizen (it was blue) he said he obtained the card from marrying an Austrian woman. My brother then said "Who do you want us to fight with? they're all killing civilians". The guy then said " The State [Islamic State] is our only hope for Syria". At that point I knew that it was a bad idea to argue with this guy. But my super smart brother thought otherwise, he started debating with him and tried to convince him that ISIS's ideology is wrong and their understanding of Islam is completely wrong. As we were walking towards Istiklal Street, my brother started bringing up verses from the Quraan that condemn the killing and executing of innocent civilians just because they don't believe in Islam. The guy said "You should read the books of Imam --some Arabic name-- and Sheikh --another random Arabic name--" he also said " Executions happened all the time in the Islamic era for infidels" My brother said "I'm an educated man and I know where to get my sources from I don't need those books," "Everything clearly is written in the Quraan". The guy then got very mad "What do you mean," he said with an angry voice "I'm not educated?!" then he pulled up his pocket-knife just a little bit so only my brother and I could see it "speak respectfully or the slaughter will begin". It was extremely crowded there but he wasn't shouting or making a scene. At that point, I had to interfere to calm him down and stop my super smart brother from saying super smart shit. "Please just leave us alone, we don't want any problems," I said, then we started walking faster away from him but luckily he didn't stop us (I don't know why). We went to the police station that day and my brother told them everything and we gave them info about the location from when the conversation started so they can see it on the cameras (it was close to the German embassy). They said they're going to investigate but they didn't really do anything. That day I felt like I was being spied on everywhere I go. I was so paranoid, I couldn't sleep the night. But thank the Buddha we never saw that guy again.