If the person has alcohol or drug dependence it would invalidate his medical certificate which would prohibit him from flying. I started flying at a very young age when drink-driving was not viewed with the seriousness it is today. Many pilots would consume 4 or more pints in the evening and drive home undoubtedly above the legal limit. Those same pilots would absolutely refuse to even taxi an aircraft after half a lager. Very few pilots actually own their own aeroplane, and even less have a private airstrip.
Most pilots hire their aircraft.
Even if the aircraft is owned by the pilot, that pilot will usually interact with several airport staff prior to taking off - any one of whom would take action if they suspected he was drunk. I would not like to land an aircraft after a couple of pints though. Even then, the most likely outcome would be a bent aeroplane rather than any serious injuries if the pilot is experienced, and even if there is a bad accident, there is relatively small danger to people outside the aircraft. Pilots usually have to regularly report by radio to air traffic control. If ATC detect something wrong in how the pilot sounds or responds, they will alert staff at the destination airport.