New Theory after Episode 8.
The Deity said that all the humans are like flowers, and the flower in her pot grows when Doom feels more pity for someone else than he feels for himself. But in Episode 1 she also said, it is his job to pluck out the flowers that have gone bad. Doom is a good "person" at heart, but in order to do his job he got drowned in a pool of self-pity, where he just hates having to what he has to do. There is also this theme of destruction is needed for new things to grow, so it is not like he can just stop existing. I think the arc for Doom is that he will become a more compassionate form of Doom, as in finding better ways to distribute destruction than just randomly destroying things. Maybe he will also find some purpose in what he does, which he has kind of lost. I think there will definitely be some sort of self-sacrifice, because for the first time he is working his powers to save someone vs. destroy someone. Maybe he will learn to love himself through Tak Dong Kyung and that will also stop the self- pity... something like that.
I think Tak Dong Kyungs arc is to do stuff strictly for herself, to learn what she wants and to just go for it selfishly, which she learns from the rather not so selfless Myul Mang. She is very defeatist and level-headed, but clearly there is a plotpoint about her never crying = not burdening others with her pain. I think she will be shown a lot of love by all her friends and family and learn to love herself through Myul Mang. I think she will want to live because of all the things he can give and show her.
Anyhoo, I still predict a happy ending. I think they will end up making the flower garden a more fair place together. Maybe the Deity is head-hunting Tak Dong Kyung for a new position, because it doesn't seem like she is against the relationship at all, but keeps egging Myul Mang on. I remember there was something about "forging their love" through suffering.