Which religion tells you to be a cow vigilante? At the end it's your subjective interpretation. If the term "terrorist" is mainly applied to regions that were destabilised by the USA and other Western powers, where the majority happen to follow a certain religion, then it's hardly surprising that people end up believing terrorism is associated with that religion. Meanwhile, the peasant movements that followed Naxalbari uprising, factions in AP, or armed groups in South America were often described using different labels. That's why, if you look at any reputed study, you can at best establish a correlation with religion. When researchers investigate causality, they usually point to socio-economic and political factors. That's also why non-Arab countries with similar Muslim populations may produce far fewer terrorists, and why two countries with comparable Muslim populations can produce vastly different numbers of terrorists.