@Kuraiaku
Well... the two exemples you give in your argument don't confuse Grim Reapers and Shinigami...
Death Note just gave it his own interpretation, without borrowing much from other cultures, and Bleach is actually fairly faithful to the original myth (the shinigami wear traditional Japanese clothes and use traditional Japanese weapons).
A good exemple would have been Soul Eater, with the boss of the academy being called Shinigami-sama... but he clearly is an amalgamation of a lot of Death myths, some even from old continental america.
In this story, the author isn't borrowing from other culture, he is clearly using the wrong myth completely.
Also, for the Japanese chuuni audience, calling him Grim Reaper (or something like "グリムリーパー" or "Gurimu Ripa", as it is already used in Japan) would probably be more impactful.
By the way, I've been reading manga for more than 20 years, and have read probably more than most, so it's not my misunderstanding of the culture that made me comment, quite the contrary.