Two words that should fill you with horror: grocery store.
Limited ingress, limited egress...clear, easy to control lanes of fire. By my estimation, which counts for as much as you pay for it, a team of less than ten armed jihadis with a van could scour a grocery store of anything breathing and *escape* in WAY less than five minutes.
Or a movie theater. One Jihadie per auditorium. No guns required.