Stupidity: Patrick told Dwayne to get Cisneros to kill Zara, apparently because he believed Cisneros could get the job done. Cisneros went into Zara's house with his gun ready to shoot, but not before pinpointing Zara's location. He saw the refrigerator door open yet entered the room without looking left and right (behind the door); Zara was able to disarm him. Zara was also able to escape Cisneros' repeated crazy swings at her with a samurai sword. Zara fatally "sliced" his neck with a pizza cutter. (00:21:44 - 00:23:07)
Stupidity: Dahlia, fleeing her husband, asks Parker, "Well, where we [her and daughter Ashley] gonna go?" Parker replies, "I know a place where you can go" and takes them to Marcus' house, parking his car there. (Mikey and Louis are looking for his car and find it.) Parker, though recently fired, is/was a social worker. Social workers are among the professions that have the greatest awareness of available community resources. Parker should have taken them to a shelter or other safe place. (01:06:04)
Stupidity: Patrizia Reggiani is no rocket scientist in this movie, but it's quite bizarre that calling the fortune-teller she sees on TV, with her name written in giant letters in two different spots on screen, she asks her "What's your name?" It makes sense for the international audience who may not recognize the word "Pina" as a name, but an Italian wouldn't have any doubt. (00:41:05)
Stupidity: Ben is standing behind Marley with a gun to her head as protection against Aaron, who is pointing a gun at him. Aaron shoots Marley and Ben just stands there, first looking at Marley on the ground then looking at Aaron (standing about ten feet away). Aaron screams and shoots Ben in the abdomen. After Ben falls, Aaron shoots three more rounds at him but misses. Ben is also able to stand and walk around despite his abdominal wound. (01:17:30 - 01:20:40)
Stupidity: David and his partner Cal drive into a weeded area near the garage/warehouse the drug bust is going to be made. David says, "This might go south. We need backup." David and Cal sit in the car talking about not letting the product get onto the street where someone might die, etc. then exit the vehicle without calling for backup, which could have been done in a few seconds (and should have been done as soon as David said they needed backup). (00:02:10 - 00:02:37)
Stupidity: To quietly kill the hotel valet he is replacing, Diabolik throws a knife at him from distance, hits him in the back, and then drags his body in the middle of the street to the nearest manhole, leaving a long streak of blood he does not conceal. If this sounds pretty risky and unsafe especially for a meticulously planned murder, you have also to consider that in the comic book from 1963 this movie otherwise faithfully adapts, Diabolik did nothing this foolish; he quietly killed the guy strangling him next to the spot he'd dump his body into.