Wait...I thought the Scranton Strangler was in jail and stuff. Didn't Toby serve on his jury? And didn't he also say it was a guy?
Doesn't seem like it could be anyone from the cast - they'd have to be in jail (i.e. written out). David Wallace is the best bet I can come up with - he kinda went crazy in season 6(ish). Wait - Roy would also be a pretty great option: Out of the series, violent temper, bottomed out, etc...
I'm going with Roy.
If you can't draw a crowd, draw dicks on the wall.