@historia Fan Xiao already was living a life where every living minute felt like punishment to him and this was long before he met Shulang. It doesnt excuse his behaviour but explains it. You think he wasnt punished enough but mentally he was already getting punished even before he met Shulang. Also, morality isnt black and white.. there's the entire grayscale in between! The hurt Fan Xiao caused Shulang were all mental wounds which he fixed one by one. He atoned for his sins. He was hunted and almost burned alive and his mental health was already a mess since childhood. So I dont know what else would have satisfied you. He suffered enough and the show explains that throughout the 15 eps without explicitly explaining it..
"for where there is kindness there is goodness and where are is goodness there is magic"