"the Slug" is who Molly keeps referring to.

As far as who's killing the old heroes generation...they eluded to it being him...but I don't think so. Slug is the one that see's Molly whenever she see's him.

I honestly believe that either Peter or a version of Hiro or someone they brought through the rift is killing of the old generation. I know that sounds crazy but I purchased something from someone off of ebay that was a set staffer & they were telling me about the Hiro rift deal.