Look, Martin Luther King, Jr., and Fred Phelps, he of Westboro Baptist Church fame, are both Christians. One is an exemplar of humanity, the other an ignorant hate-monger. We might not like Phelps and his kind, but he's got his Bible verses. So arguing from scripture about what is or isn't the "correct" interpretation of a religious tradition is a non-starter. Nor is it effective or advisable to argue about God's intention or what God wills. As long as you're a human being, you can't know. You can make a guess or even pray for guidance, but the ball is still in your court.
So we need to argue from extra-scriptural sources, namely life itself and our judgments gained from our experiences. It's up to us to say what we want to emphasize in a religion and what we want to jettison. Here are a couple standards to start with. First, if your religion tells you to kill in God's name, it's wrong. I don't care what the scripture says. Second, if your religion makes it more rather than less difficult to deal with peacefully-expressed differences of faith, culture, and ideology, then on that score it's wrong.
To a fundamentalist, these propositions are anathema. But a lot of non-fundamentalist or non-fanatical believers also have a hard time with this, even when the conclusions they are drawing about their religion are not hateful or vile, of the type we see with ISIS. I guess that some of these are people that worry about offending God's will, and thus the attempt to argue from scripture about what God intends. Moderate to conservative religious believers must learn to acknowledge that it is okay, even good, for us as human beings to make conscious judgments about our religions in the name of greater peace, harmony, and well-being for humanity.
Actually, let me amend my argument, and quote some scripture. "By their fruits you will know them." When a religion enriches the world, and brings people closer into brotherhood, those fruits are keepers. So no, ISIS hasn't "hijacked" Islam, but they practice a form that has no place in our world.
This article by Graeme Wood in the Atlantic is a must-read for understanding the religious beliefs of ISIS. Among many other things it discusses how they are an end-times, apocalyptic cult, which, by the way, is a characteristic of many American Christians.