That is definitely Luke's voice.
I have a feeling Luke is going to realize the failure of the original Jedi code about not allowing love and it's hypocrisy. Remember, the jedi Luke knew, Obi-Wan and Yoda, said Vader could never be turned, but Luke managed to get Vader to fulfill his destiny and defy the Dark Side.
Perhaps Luke adopted the old Jedi teachings and realized their flaws indefinitely due to Kylo's betrayal, and is deciding to abandon the Jedi Order forever. Instead, he'll focus on teaching more grey morals of Jedi, and emphasizing the importance of love and compassion. Maybe that's what Luke needs to teach now. It's evident Kylo never learned the importance of family due to murdering his own father. This makes me feel as if Luke never taught his students such important lessons of love, and thus failed.