if your subletter turns out to be a psycho, you are basically fuq'd. As the person who signed the lease you remain the one legally responsible for the apartment, and if your subletter stops paying you rent or trashes the apartment or turns it into a crackhouse you are still expected to pay the rent and damages.
although this all seems unlikely, there is actually an incentive for a subletter to stop paying rent, simply because you have no legal grounds to force them to, since the entire arrangment is illicit.