Actually, the I Rex wasn't as much a bad guy, but rather a victim of circumstances. Think about it: It was alone it's entire time. Isolated and caged. Then, it tasted freedom. And the first creatures encountered outside of it's enclosure, the ACU members, attacked it. So, it's quite natural, that the dinosaur assumes, that everything is hostile. Owen might have said that "She kills for sport." but i think it's more accurate to say that she "kills as an preventive measure". After all, the Indominus never had a chance to actually learn what would harm him and what would not. And let's not forget, that beeing agressive is more or less hardwired into this creature. With all the shit that happend to it, it can't act any other way.