What An Idiot / My Hero Academia: Unchained Predator

• Nine, Mummy, Slice, and Chimera see that the Slayer is nearly done slaughtering all of the Sabers.
  You'd Expect: Them to run. While they are skilled at fighting, so were all of the Steel Sabers, to the point of being able to kill 246 different heroes, and they're all dead. While it could be argued that the four are more powerful than the quirkless soldiers, the Slayer has thus far made mincemeat of almost ten thousand soldiers actively trying to kill him with guns, missiles, bombs, and tanks and has thus far walked through them as if they were nothing to him (which they were). In fact, many of those kills were with his bare hands, and the streets are literally running red with rivers of blood from how many people have been reduced to something with the consistency of ground beef.
  Instead: They decide to fight the Slayer head-on and are promptly murdered with extreme prejudice for their troubles.
• When Endeavor arrives to the island in Chapter 20, showing outright hostility and anger at the heroes for not capturing the Slayer, he demands that they tell him his last location.
  You'd expect: Them to tell Endeavor to stop behaving like a raving lunatic. While it can be justified that the Slayer running free is a problem, the bottom line is that the Slayer had made mincemeat of ten thousand soldiers and royally trounced Nine and his lieutenants, and that charging headfirst would be tantamount to suicide. Not to mention he was terrifying the hostages, who had been through enough already.
  Instead: Ryukyu blurts that the Slayer's last known location was the central tower, and that Hawks went there and was about to explain that they haven't heard from him yet when he already left.
  The Result: Endeavor flies there, catches the Slayer in the main vault and unleashes a slew of attacks on the Slayer, including his ultimate move. It didn't work, he destroyed the vault, and the Slayer brutalizes him in a one-sided fight. Recovery Girl sums it up in Chapter 27.
  Recovery Girl: It'll take at least three months for him to heal. It would've taken six months if I hadn't used my Quirk.
• When the heroes on I-Island debate on what to do in the beginning of Chapter 21, they don't know what to do regarding the Slayer due to a lack of intel and the possibility that the man is Quirkless.
  You'd expect: Them to ask the last known people who spotted the Slayer, which was Izuku, Ochako, Shoto, Melissa and Mirio. Even if this would be a problem for the heroes to ask five traumatized teens, the five teens know that the Slayer's armor can shrug lava off like it was nothing. It would enable them to come up with a plan, or at the least, realize they need All Might or someone of his strength caliber to fight the Slayer.
  Instead: They don't consider the possibility and with the exception of Slidin' Go and four others, rush off to fight the Slayer.
  The Result: All the heroes receive a non-lethal Curb-Stomp Battle for their failed efforts to capture the Slayer.
• When the Slayer emerges after his fight with All Might with tiny dents and a fist-sized dent from All Might, everyone is left terrified and stunned speechless as the man is basically undefeatable.
  You'd Expect: Them to not do anything. While doing nothing would mean cowardice, the problem is that the Slayer has, in the span of one night, singlehandedly slaughtered the most infamous mercenary organization, viciously ended Wolfram's group, forced Curator and the Volcano Thieves to retreat when it became clear to them that fighting him would be suicidal (though the heroes arriving from the mainland and the ballroom heroes being freed played a part in that), knocked out 48 heroes (nearly 20 in the Top Ten of at least 13 nations) and bested All Might in combat. Not only that, but even with All Might levels of strength, none of them stand a chance against the man, as even Miruko views attacking the Slayer as downright suicidal and moronic.
  Instead: Bakugou stupidly tries to fight the Slayer, despite the fact All Might got bested by the man.
  The Result: Bakugou gets predictably captured, forcing Miruko to try and save him... and get captured as well.
  Even Worse: When Bakugou tries to use his Quirk, it's despite the fact there's a leaking gas tanker and the area surrounding the Slayer was covered in gas. Had All Might and Miruko not screamed for him not to, the Slayer would not have noticed and hurled them away until it was too late, and society's views of Bakugou, had he killed the Number 1 and 5 Pro Heroes of Japan to stop a man not even All Might can beat, would be as a future villain in the making, provided the Slayer doesn't reduce Bakugou to a mangled and mutilated corpse because of that.
• In Chapter 18, Ragdoll's usage of her Quirk on the Doom Slayer has rendered her permanently insane (until Jeremiah removed those memories 10 chapters later), and Miruko isn't aware of it.
  You'd Think: That Hawks would tell her while they were leaving the medical facility of I-Island, or in Rumi's hospital room, or literally when they and the Slayer were fighting Wolfram. The only reason he doesn't reveal this to Sayaka and Mera was because four teens were here for the meeting (that being All Might's skinny form), and finding out one of the heroes went insane by reading the man's mind would be a bad idea.
  Instead: He doesn't.
  The Result: Miruko nearly gets an eye gouged out by a completely insane Ragdoll.