I think the point he is making is that this event should not be used to call anybody who disagree with Islam is responsible for this, like basically a Christian cannot theologically be against Islam and see no truth in it and feel Islam, as a religion is generally an anti-Christian one, because in doing so he is of the same mould as the terrorist (pretty much making it impossible to defend Christianity and by default makes Islam the "true" religion. But in reality a Muslim who thinks Christianity is corrupted and that Christians are guilty of "Shirk" should be considered the same as an ISIS member, if fair is fair)
A good example would be to say, that if an Islamist carries out a terrorist attack, any Muslim with critical feelings about western foreign policy should also be called an extremist, as Islamists use foreign policy as a big excuse, so a Muslim who simply thinks the Western powers should stop adding petrol to the flames in the middle east, is an extremist, its a silly notion, there is legitimate arguments against foreign policy.
Those people at the mosque were murdered by a extremist today and such actions are vile on any human being.