Henry Peverell had two goals when he took his one way trip to the past. Eliminate the Dark Lord Voldemort by destroying his Horcruxes and kill his grandfather, Harry James Potter to prevent the rise of even greater Dark Lord. He figured acting as the History of Magic teacher would give him the opportunity to accomplish both...he didn't count on getting attached to his grandfather, nor did he count on falling in love with a woman as skilled at curse breaking as him.