I'm sorry if I'm coming off as a bit rude. Rape is just a very overused trope; I mean it WORKS to sell how despicable someone is, but I think it would be better if the rape was a minor thing compared to how much this guy kills for the THRILL of it. Doesn't matter who: Man, Woman, Child--they'll be killed because it gives him a rush he can't get any other way (other than rape) and he carves a path of destruction wherever he goes. The only way Yoko probably got him to be a distraction is by telling him he could kill, but I think Yoko believed the rumors about him might have been overdramatic until he actually SEES what this guy is capable of. 

Yoko tried taking him out, believed he succeeded but didn't, and then just left it as is. The fact he isn't wreaking havoc in the world may not only be that he's still weak, but that he may be trying to force Yoko's hand to confront him where he is strongest, and it's likely to be a place absent of life to put Yoko in a weaker position. That, OR he has a subordinate making these suicides occur, and the subordinate is just a mid boss for who he's REALLY working for. What do you think?