It was the last time, that he heard the name that his parents have given him, when his mother has given birth to him - Ben.
Ben his weak and foolish former self, that he thought he has destroyed forever. Moments before, his father stood in front of him and now he's gone. Gone because he has murdered him.
Kylo Ren still feels the hand of his father caressing his cheek, when Han Solo's dead body falls in the abyss.
And now there is it again, his former self Ben. He suddenly feels sorrow for what he has done, instead of relief from his inner pain. Tears well in his eyes. Tears of regret.
Supreme Leader Snoke has warned him to give in to sentiment. It was once his grandfather's undoing. Kylo doesn't want to make the same mistake.
But the little part of Ben, which survived in him, remembers the rare times together with his father aboard the Millennium Falcon. They were full of joy and adventure.
Since he has betrayed his uncle and the new Jedi Order he is torn apart. Conflicted between the light and the dark side of the Force.
He thought killing his own father would be the test, that he has to face, which Snoke mentioned. But there is still the pain and he still feels torn apart.