It would actually be so pathetic if he did that to all the innocent women but didnt even take revenge on those people that hurt his sister.
I wonder if he even knows that she was murdered or if he actually believed the suicide story. If he thinks that she actually committed suicide I can see how he can turn out this way. If his sister was suffering so much that she felt she had no other choice but to take her own life I can see how he wouldn't want that pain for any other young girls and the way he kills them is pretty humane as far as murder goes. I'm pretty sure most people wish that they would go in their sleep anyways. His actions are him trying to make up for the fact that he couldn't save his sister but he's just doing it in the worst way. There so many other things he could do to help those girls.
Though in my opinion driving someone to suicide is just as bad as murder, so the adoptive family would be an object of my revenge anyways even if I bought the suicide story. That family and the families of all the other girls needs some form of punishment. But then again, this dude obviously has a few screws loose and might not be thinking his actions through fully. Grief can do crazy things to people, but it doesn't justify his actions at all.
You think you're so good and special, when all you are is just another pathetic nutjob using innocent women to vent his own personal issues.
And I still wonder... is this all he did? ...Did he or did he NOT also get revenge, on the ones that *actually* killed his sister?