To me, this was a powerful story of free will and choice even when the odds are all stacked against you.
As I see it, Tan Tai Jin is not evil, and surprisingly sane sane if you look at his circumstances.
When we first see him he is getting tortured - which, as we soon find out, is his normal way of living. And he looks at this with a stoic resilience.
He is said to be evil from birth, but though we hear this repeated over and over, it is not what we see.
In a way, his story is similar to that of Wei Wuxian in the Untamed, where he remains evil in the eyes of the "people" regardless s of what he does.
But in this unforgiving and bleak life of his, Tan Tai Jin gets help in tbe most unexpected way, when Lin Susu is sent back in time to kill him. While in theory, this is just her mission, she can't help but start treating him in a decent, humane way, which TTJ first cannot really think with, as it never happened to him before. But as time goes by, Susu becomes the one ray of light in his life that he holds onto and that ends up changing both of their lives like no one would have expected.
I think this is a great story very beautifully filmed, with lovable characters. Ultimately, despite all the pain and trials this is a story of the power of love.