Obviously the title of this thread is pretty self-explanatory. Who do you think wrote the note? My personal vote: Andrew.
it may sound weird but think about it: He lives in Martha Huber's house, so it would bring back that element from Season 1. When the show started Bree had problems with Andrew. What better way to have that relationship come full circle than to have bad Andrew come back one last time? His motive? He blames Bree for his alcoholism and Alex leaving him. At some point the wives find out that it's Andrew and we have a final showdown with Bree and her son, resulting in his death....at the hands of Bree.
again, that may sound extreme but hear me out. Marc Cherry's initial inspiration for the show was the Andrea Yates trial, the woman who murdered her kids. He said to his mother "Can you imagine a woman being so desperate she'd murder her own children?"...well, in this case Bree would definitely be desperate enough if he had his hands on this information. it could even happen halfway through the season, so we see the ramifications later. Bree would be conflicted: relieved that the secret is still safe, hurt that her son was the blackmailer, devastated that he's dead and guilty that she did it. this could bring another bout of alcoholism or another stay in the psych ward, or even having her end her life in the series finale (if in fact, a housewife dies in the finale) she could be welcomed by Mary Alice, Rex and finally Andrew and the two of them finally forgive each other once and for all and the both of them can rest in peace.



He may have found out some evidence and traced down Bree ? I think I need to stop with the crazy thoughts now 
