You IDIOT!!!

[Trevor]

Not a movie, but my favourite TV episode that fits this is the Teso Dos Bichos episode from The XFiles where people are being killed because a sacred burial urn was dug up and sent to the USA.

The idiotic part of it centers around the victims continuing to hang around the museum where all the killings take place instead of leaving it. 

[BTM]

Pretty much the entire cast of Feast 3. 

"Hey, let's go into the sewers, that's a great idea!"

"Hey, there's writing in blood on this wall... hmm.. let's keep going." 

[3mnkids]

Autopsy~  The entire cast is stupid and I was rooting for the bad guys. I know if im in an car accident in the middle of nowhere and an ambulance just happens to pull up and two guys get out that look like they just escaped from prison and they proceed to pick up the body off the ground and roughly throw it on the stretcher i will not be getting in that ambulance.  seems like common sense to me. 

[Olivia Bauer]

Almost every horror movie character ever. But what really takes the cake is in Quarantine when they are surrounded by things they can use as weapons... That they don't use. As I recall one guy was HOLDING an axe. He used it once! ONCE! Then he never uses is again. What kind of idiot only uses an axe once when being attacked by a force that tries to kill him?! 

[Dennis]

This is a generic "B" movie character, usually a scantily clad, very pretty young woman who, even though she's aware that a killer/alien/slobbering monster/mutant is loose, hears a strange noise and goes to investigate armed only with some sort of household utensil. An example is the woman in "Sabretooth" who is aware that people in the area are being killed and eaten by a wild animal yet when she hears someting in the bushes outside her cabin goes out to investigate wearing only her boyfriend's T-shirt and carring a coffee cup. 

[InformationGeek]

Harper character in Megalodon appears in my mind.  He was suppose to remain quiet and turn on his minisub while the giant shark was circling around.  Everyone was telling him to not do anything, but he still turned on his sub and was promptly attacked and kill by the shark.

Beware!  The Blob suddenly pops back into my mind as well.  The sheriff did abosolutely nothing throughout the film besides act like a jerk and ends up apparently taking credit for the defeat of the Blob.  I presume the Blob ate him when it started to melt.  There was also the Chester character who brought back the Blob in its container and left it out in the open, instead of leaving it in the fridge.  The Blob, aka the killer jelly, ate him up.