Okay, so Han knew enough about the Force to know that Kylo had some good left in him. He also knew that if Kylo killed his own father, what little is left of the good in him will be destroyed. He also knows that if Kylo doesn't kill him, Snoke will kill Kylo for failing. So he killed himself to save his son. Think back to the scene. "Help me have the strength to what I have to". Who looked more suprised when Han died?