Stupidity: When Joan is in hell and is confronted by the overlord, she attempts to shoot him with the BFG-9000, so the overlord uses his powers to telekinetically blast Joan away and disarm her. A minute later, Joan picks up the BFG-9000 again, but the overlord never considers using its powers to blast Joan away from him again, which results in him getting killed.
Stupidity: Joan arms herself with the BFG-9000, and she, along with Bennett walk up to an elevator and wait for it to arrive. Once the elevator arrives, Bennett is standing with his body facing towards Joan and is facing away from the elevator. Considering the compound is filled with monsters and demons, it is extremely foolish of Bennett to not face the elevator, or at least be prepared, when it opens. This stupidity results in him getting captured.
Stupidity: When the marines get the reactor working, Dr Betruger stabs Veronica and kills her so that she will not be able to reveal his secrets to them. However, he stabs her in the lower back, so it would still give her ample time to tell the marines what he is trying to hide whilst she bleeds out. Surely it would have been easier to stab her in the neck or slit her throat.
Stupidity: When the overlord confronts Joan, he summons hundreds of demons to make sure that she is cornered and can't get away. However, the demons only appear to approach Joan from behind and from the side. After she kills the overlord, there is an entire area in front of Joan (behind where the overlord was standing) that is completely free of demons that she is able to run towards and escape. If the overlord was intent on cornering her, he should have made sure that all surrounding areas were covered.
Stupidity: Whilst the chaplain and one of the marines are talking in the kitchen, a demon sneaks up behind the marine, stabs him through the back and kills him. The demon then walks up to the chaplain and strikes him across the face and leaves the room to search for the rest of the crew. Despite the intent being to kill the crew, the demon does not make sure that the chaplain is dead before leaving. It just assumes the strike across the face will be enough to kill him.
Stupidity: When Joan and Winslow are walking through Level 2 after the monsters begin attacking them, at one point Joan turns around and shoots at a monster, but she misses and the monster is able to hide. After this she turns back around and continues to walk down the corridor. The monster she shot at isn't dead, it's just hiding, but instead of going to inspect where it ran off too, she decides to walk in the opposite direction, putting herself at risk of the monster sneaking up behind and attacking her.
Stupidity: Just after Joan finds Winslow after he kills a monster, they discover that it was actually a human geneticist, Dr John Carmack, that transformed into a monster, and they realise this by checking his name tag. Even if they didn't know the extent of what happened in the compound beforehand, the monster was still wearing human clothing and its body was still in the shape of a human body, so they should have, at least, realised that it was originally a human, even before looking at the name tag.
Stupidity: When the marines are searching through Level 2 and detect lifeforms, they are about to shoot, which prompts a surviving scientist to run out into the open shouting "Don't shoot! Don't shoot!" This was a very foolish way of trying to get them to not shoot. Running out into the open like that could have prompted the marines to just start shooting out of shock or instinct even if she was going to say not to.
Stupidity: Once the marines have climbed down into Phobos compound, and have entered through the airlock door, they check the control gauge to see if the O2 levels are good. They were aware this was something that needed to be checked, but they brought no protective equipment with them in case the air was unsuitable to breathe.
Stupidity: The marines all wear helmets as part of their protective gear when they go into the Phobos compound, but Bennett and Akua do not wear any helmets at all. They wear bullet proof vests and arm themselves with weapons, but unlike the other marines, they go in without any way of protecting their heads.