What An Idiot / Battle Royale

Warning: Since Too Dumb to Live is in full effect here, expect spoilers unmarked. Read at your own peril.

• When Mitsuko is caught trying to kill Tadakatsu, he decides to hold her captive for a while and make sure she doesn't try and kill him.
  You'd Expect: He continues to keep an eye on her due to her terrible reputation that he openly knows about.
  Instead: He not only trusts Mitsuko, but tries to screw her (despite her reputation).
  Yep: He actually starts off suspicious of Mitsuko, but once Mitsuko does her thing...
  As a Result: He gets a sickle to the throat.

• Hirono also gets one in the film. She holds Mitsuko at gunpoint while the latter has no gun.
  You'd Expect: Hirono kills Mitsuko quickly since she knows how dangerous she is.
  Instead: She kicks her while her down and spends so much time doing so she ends up getting zapped by Mitsuko's stunner and then shot dead with her own gun.

• Toshinori Oda in all versions receives a bulletproof vest, and is gunned down by Kiriyama but survives.
  You'd Expect: He'd simply play possum and wait for Kiriyama to move along (and, to be fair, he DOES do this in the novel, but Kiriyama goes back for a Double Tap).
  Instead, in the film: After Kiriyama leaves, Toshinori actually shouts out "I survived thanks to my awesome bulletproof vest!"... at which point Kiriyama returns and beheads him with a sword.
  Instead, in the manga He fakes a death rattle in the hopes that Kiriyama would approach to check on him, at which point he'd shoot Kiriyama. When Toshinori springs to life, Kiriyama quickly jumps between Toshinori's legs and fires a volley of bullets into his unguarded dick. For extra points, Kazuo was intent on chasing Hiroki down at the time so he could easily have escaped.
  For some reason: He is only called out on being an idiot in the novel, where the Lemony Narrator says he should have remembered that Kiriyama confirms his kills, despite his actions in the novel being the most logical in comparison to those of his other versions.

• In the manga Hiroki manages to shoot Kiriyama seemingly dead.
  You'd Expect: Hiroki of all people to remember that Kiriyama has a bulletproof and headshot him; after all, he was there when Oda was killed (at least so he thinks).
  Instead He forgets and turns his back to Kazuo, and you can probably guess what happens next. (To be fair Hiroki was under serious stress at the time, having just lost an eye and some fingers).

• Mimura has a plan to hack into the computer system of the school and disable the collars, then proceed to escape from the island, which he tells to Yutaka. The plan fails when the attempt is foiled and he realizes that their collars are bugged for sound.
  You'd Expect: Mimura to realize that a paranoid government that made sure to explain to them that there is no escape from the island or game would also keep tabs on the students not just by the collars, but also via sound.
  Instead: The plan fails; they think up a more dangerous, but also more direct plan on just bombing the school and escaping that could very easily go wrong. The plan would've likely worked anyway, and Mimura does acknowledge the collar bit in the film, but Kiriyama came across them before it could be pulled off. Thankfully, Shogo, who already made the system vulnerable long before, manages to disable it fully later on.

• So the respective teachers in the Novel and Manga realize that Shogo lied about shooting Shuya and Noriko, and that they are still alive. Then, he decides that Shogo must die and points a gun at him.
  You'd Expect: He shoots him immediately. After all, Shogo managed to survive the previous Program, so the teacher should know how dangerous he is.
  Instead: He monologues to Shogo for a bit while pointing his gun at him and later gets distracted by some gunfire, giving Shogo an opening to kill him with a pencil.