For the torture sequence, you don't have to kill the suspect, you just have to break him badly. He does however, get back to life with an adrenaline shot every time you kill him accidentally, so don't worry about it. A good tortue combination is using some nice little shocks, then some wrenching, then a tooth reposition, then electrical leads and finally more shocks.
Once you're done, kill him, change to Trevor and take him to the airport.