Before I saw Tuesday's show I was thinking Kai... because he would not be someone who would be too obvious. He was in the bedroom so it was not him.
My current not to obvious guesses...
Justine Turner - she knows he is scum because she overheard him at Sonny's and is frustrated because she can't do anything about it legally?
Molly Lansing - she has been through a lot... she always ends up taking things better than most and being rational... she hit her breaking point?
Martin Grey - he feels horrible for helping Drew for so long... he has no kids... no wife... so no one to leave behind... he wants to take one for team and be rid of Drew?
Stella Henry - he has messed with her family in every conceivable way... she thinks she is going to prison anyway... why not take out the trash first?
But... I will not be surprised if it gets blamed on several people before we know who it really was.