I do think you want to treat it as "murder is a serious crime and needs to be prosecuted and punished" rather than "this is an act of Islamic terrorism and we need to declare War on Terror!". But, universally ignored is hardly how you want to react. You want to close borders and you want to declare emergency and you want to, ideally, apprehend these people using conventional police. You do not want to live in a permanent state of panic or build a police state after the fact, and you definitely don't want to take out your anger on a whole population of people over the actions of a small radical group. But you do want a, rational but concerned, reaction.

Right now, you want to get people to safety, help the survivors and catch those responsible, in that order.