That, to me, falls entire to the religious leaders of said religious groups, who can influence followers of the religion. Sadly, in the case of ISIS, it took the Muslim leaders way too fucking long to finally denounce ISIS as not being Muslims and it is only after ISIS burned a Muslim pilot did they decide to do that. They're hypocrites in my eyes for remaining silent on the issue until another Muslim got killed publicly.
Still, my original post primarily focus on what I was taught and yeah, I was taught that murdering people for a selfish cause immediately disqualifies you from being a Muslim. But as I've said, the ISIS terrorists believe they're Muslims and kept preaching about it so it is difficult not to see them as an Islamic group. Still, I'd like to believe that intentions are more important than faith, and if they intend to "cleanse the influence of the West" through violence and take over the world, Muslim or not, they're not earning brownie points with me. They're a threat and must be eliminated.