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Boys Next Door

Avatar by tassyn.livejournal.com

Reading: Kingdom, Sangokushi, Historie

BL: A to Z
Doesn't hurt to check out the murder + yaoi tag genre search:
http://www.mangaupdates.com/series.html?category=Murder&genre=Yaoi&perpage=50]

Simply another normal human being...

Essentially giant spoilers for the two manga below:

While the following recommendations don't necessarily have characters who can be tagged with the label of "serial killer," the main "antagonists" kill a ton of others. When I say a ton, I mean a ton.

Zankoku na Kami ga Shihaisuru The people drop like the end of Hamlet in this one. The person who does all the killing seems to have the basic psychology of a serial killer (and this manga is deeply psychological as well), so it could be what you are looking for.

Sakura-Gari Lots of murder here, too, but the mentality of the killer(s) is different, so it might not be exactly what you want (unless what you want is a lot of death, which this one definitely has!).

It suits your discription: Mask
Well, vol.2 of Crystal Harem will do as well.

New York New York

it takes time to get to that point and it might not be what you're looking for, but still a good read.

thanks for all recommandations ^_-!