Light Yagami from Death Note. I mean, just a couple days ago, I started Leggere DN, expecting Light to be the good guy from all the fan he has, but now that I'm on the secondo volume, I see that he's friggin COLD BLOODED! He's talking about testing the DNB on complete strangers, and he went from just placing justice on the world da killing criminals to killing anyone that gets in his way of what's short of ruling the world! He's manipulating women's emotions, using his sleuth skills, and even talked about killing his family just so that he'd "win". Frankly, I'd rather hang out with Ryuk than that creepy jerk any day. 8-/
Hmm..well for me that has to be Dark Marik from Yu-Gi-Oh!..He can get really scary at times! especially in the Original (Japanese) version of the anime..twice as scary in there..in there he actually wanted to KILL Yugi-boy..he really can get quite scary at times.