Throughout act three, I see the action spiralling out of the control of any one character.
Near the beginning of the act, Ophelia and Hamlet have a falling out... Ophelia attempts to give back Hamlet's tokens of love, and Hamlet gives contradictory messages about never having loved her. This interaction kind of hints at the way things are going to end for the two of them.
Hamlet is trying to manipulate everybody by getting the players to add his scene and freak out the king. The king is trying to manipulate everybody, too, by trying to get G. and R. to figure out what's wrong with the prince. Despite their attemtps, things quickly go downhill.
The king loses control when the play accuses him. He rushes out of the room, clearly upset.Hamlet's feeling pretty good at this point-- he's about to get revenge on his evil uncle. He even gets the perfect opportunity to do so, but hesitates because his uncle is praying and he doesnt want to send his soul to heaven. This indicates the ending of the play, as well. Hamlet's thirst for revenge will not even allow him to murder the king if the king will not suffer enough after death. The way he holds onto the anger and desire for revenge could be considerred the flaw that leads downhill.
The situation is taken out of Hamlet's control after he speaks with his mother and murders Polonius. From this point on, there can be no changing his mind, undoing everything, and going back to normal. This simple, mistaken act has given away Hamlet's plot and created enemies for him, as well as broken the heart of the girl he loves. Other moments in the scene do start to point out where things are going, but the moment he kills Polonius, in my mind, is the real crisis.
From that moment, the play (and Hamlet) become increasingly creepy, and the reader or watcher can only look on in horror as the things set in motion begin to unfold.