The killing curse will cause anyone to be struck by it to die immediately. If the player uses the killing curse on Harlow, instead of it killing him, he's still alive and after defeating him, it's mentioned that he'll be sent to Azkaban for the rest of his life.
Inferi are known to stay in very dark places because they are repelled by anything bright like the sun. Many times during game play, Inferi will come out of the ground in daylight and attack the player and even appear in brightly lit catacombs.