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.
Stupidity: Instead of breaking into Gordo's house, covering all the furniture in one room with sheets/thin blankets, and waiting for Gordo to return home in order to kill him via a "dramatic re-enactment" (in an adjacent room which was not shown to the audience), Bill could have taken Gordo anywhere to avoid getting caught or calling the police on himself. Had Bill taken Gordo to his already-"wrapped" motel room, he had enough injuries after the fight to claim self-defense against an intruder. (01:43:20)
Stupidity: For someone trained like "H", it was absurd that he would run down the center of the road toward masked gunmen and get shot multiple times. It was also ridiculous for him to have taken his son to a location that was potentially dangerous and put him in harm's way in the first place. (01:13:20)
Suggested correction: I'm not sure how much H was trained. If I recall correctly he's a skilled criminal, and all his "training" was faked so he could get the job with the company. Regardless... training or not, all that goes out the window when your child is being attacked. Paternal instinct took over and he was rushing in to protect his son. As for your 2nd point, it's made clear that his task was to simply stake out what was supposed to be a routine truck route. They had no idea the truck was going to be robbed.
Stupidity: Why would the tow driver go around Deacon's truck in the one-way dead-end parking lot to pick it up from the front? He'd have to back it up on the hook, and Deacon's truck would be in Park, making it impossible. There's a reason tow trucks hook up vehicles by the drive axle ends. (00:08:40)
Stupidity: While awaiting their flight home, Nancy told Teri, "Think I miscalculated the tea. I'm not gonna last three-and-a-half hours [on the non-stop flight with no restroom facility]." Nancy then went into the gift shop where Stuart Couch introduced himself, asked if he could get her a drink, and bought her a bottle of beer. Nancy drank at least some of the beer on screen, even though she was already concerned about drinking tea before her flight. (00:59:28 - 01:00:30)