Keep your eyes open, wherever you are. Know how to get out. It's going to happen again. If we're lucky, it'll be more dirtbags like this; if we're not, it'll be some trained jihadi who walked across our 'secure' border, or was let in as a refugee.
Oh, and you can count on all the Usual Suspects screaming "If only we'd had more gun laws!" Because terrorists planning mass-murder are very careful to obey such laws.