The most important thing is the sudden and very noticeable change in art style from cutesy high school character design, such as we’ve seen throughout all of the game so far, to horror anime style. The lines and shadows are much more defined, and a lot of effort is made to draw the watcher’s attention to the forehead and eyes of the culprit as the focus points of their fear.
Everything is slightly out of proportion in every frame, putting extra emphasis on the most noticeable and unnerving parts of each scene. The culprit’s state of mind, their transportation to the scene of execution, the scene itself and the horror of their method of execution are given a lot of time and attention.
Notably, it’s the only execution where we see the culprit taken from the previous scene that we know and transported to the scene of execution. This may seem a strange point to make, but it’s important for instilling fear. You fear the things you can relate to, and if you can see how the culprit arrived at their execution, it’s a lot scarier than if you see them already there, because you can imagine the same thing happening to you.
Personally, I think that this execution is the scariest for three reasons:
1) It’s the most violent. Other executions are more brutal, others (debatably) take longer to kill their respective culprits, but this one combines pain and duration. I am sure that, out of all of them, it is probably the one we’d least like to suffer (perhaps tied with the second).
2) It’s the most relatable. Whereas the other executions are, for want of a better word, quite wacky (with the exception of one of the executions near the end), this one makes an effort to seem realistic. The fear that the culprit shows is real fear. They, unlike in any of the other executions, seem to be making consistent conscious efforts to escape their fate all the way up until their execution. This is an action we can relate to: it is what we ourselves would do, and it mirrors the fear that we ourselves would show in that situation. We see the full passage of the execution from when the culprit is standing in the room with all the other students, seemingly as safe as any of the others, to when they are killed. This makes the execution less particular to them: it is impressed on us how nobody in the room is safe, and that the same fate could befall any of them.
Finally (for this bit), the culprit’s facial expressions before their execution are some of the most horrifying pictures in the whole of the game. They look at the others, still desperate for a reprieve, for some mercy. After they are caught, they still find it difficult to believe that they could be guilty, that they have been judged to have murdered someone. You get to see their face as they realize that because of what they have done, they are going to die alone and with no friends. You are able to see the realization that their fate is approaching dawn on their face. You are able to see their last moments as they desperately try to escape, then realize their fate, and you watch as the sadistic host that has made it all happen make a game out of it and try to portray it as fun.
3) It has the most shock value. Let’s be honest here. This execution would still be the most horrifying if it was further on in the series, but the fact that it is placed first lends it an extra dimension of horror.
It introduces you to not only the fact that culprits will be executed, but it also shows you how they will be executed: brutally, sadistically, but also with no respect. The host expects you to laugh as well as gasp at the irony of these executions, and the realization that there is someone out there who thinks like that, and that that person has complete control over the lives of 16 high school students, is truly horrifying, and it is all running through your mind as you watch the culprit die, helpless to do anything.
The first execution is only the beginning, and with it comes the fear that no-one, not even you, is safe anymore.
NB: I hope I’ve kept this Spoiler-Free. If I haven’t, Ax has permission to stab me with chopsticks. Again.