In An American Werewolf in London, Jack notices a pentagram on the wall of Slaughtered Lamb. But it was never elaborated what it has to do with the werewolf and why it made everyone in the pub so silent. Who made that sign or what was it's importance? Was it to indicate voodoo or some other kind of magic involved in the werewolf mystery? Was that ever addressed in any extended media/sequels etc?
The pentagram, especially in the form where it is inscribed inside a circle (a pentacle), is in European folklore a symbol which repels evil spirits and demons and protects its wearers. There are a plethora of symbols and rituals which have similar purposes which can be a bit confusing for people from other cultures.
It is also an old belief (which is shared by many cultures) that once you are talking or even thinking about evil things or draw interest to something to which they belong, you are luring them to the talker because they feel that someone is talking about them, You know who I am talking about, He who must not be named, right? While the hostile reaction of the pub people is exaggerated (it's a horror movie), superstitious people will be dismissive against people who are in their view challenging fate.