I've never seen the play nor plan to. So, I just read the Wikipedia article. The Wikipedia has a brief Synopsis of Act I & II and then a section titled 'Identity of the murderer'. My guess was correct after reading the breif synopsis. Really after the twist endings of The Usual Suspects, Palahnuik, and dare I say M. Night Shamwow, this is not a big shocker, and really it's very predicable.