I feel you vastly misunderstand why an R-rating would be needed for an accurate adaption of Death Note, a great live-action death note.
Death Note has a lot of fairly graphic scenes, especially in the later half of the storyline. The death of several characters is done in an increasingly violent way, the death of Matt and Light Yagami for example features them being shot multiple times. That sort of execution-like scene alone would warrant an R-rating in most films.
That isn't even beginning to factor in all of the dark themes that this movie includes such as the nature of justice, suicides, murders, mass killings, cults, etc... - along with one of the most difficult aspects to pull off... having a serial killer be shown as a relatable character whose descent into madness can still cause viewers to question their own views on what is right and wrong.
The Death Note anime on its own warranted a 15 rating, and that isn't even live-action. A straight-up adaptation of the anime alone would warrant a R rating, let alone if they tried adapting the manga.
An R rating doesn't mean that the movie will automatically do things such as you described.