Why does Tom Ripley cut his hand in The Talented Mr Ripley?

Towards the end of the movie "The Talented Mr. Ripley", Matt Damon's character Tom cuts his hand on purpose as he talking to Marge. She's on to him about the whereabouts of ey and Freddy. But I can never figure out why he purposely cut his hand in the movie. Does someone know the ligitimate reason?