Muslims believe that if one of them is attacked by a non-Muslim, then all of them are attacked. Logically, this should mean that if one of them attacks a non-Muslim, then all of them are responsible for that action. You cannot with integrity choose to be "one people" only when it suits.
It is exactly the same for Christians. If one of our number causes harm to another person then we all share the blame. At the very least, we should respond to such maverick acts with a vehement, unambiguous and public condemnation of the crime.
Islam cannot claim to be a good thing until the majority of Muslims stop being complicit in acts of terrorism by way of their silence.