Unfortunately, but both he and the devil were in their eye motivated by love and just defending their men. The ending is quite sad for the detective and Silver haired Sealwoo, but at the end of the day they both committed crimes, obviously we know the motives of this for Sealwoo and I still think the detective being killed by the devil was unneeded but from a plot perspective, it meant things got wrapped up nicely. It also meant that a contrast could be made with the demon couple who are shown in the end in a happy domestic setting, exchanging rings etc. Guessing they had a demon version of a marriage contract.
I guess though it can make you reflect on what preventative measures could have been put in place to prevent Sealwoo from behaving like he did, but it is what it is. You can still feel sympathy for him, his "light' ie the detective is no longer in his life and he has had all of rights taken away from him, given his background it's seems Sealwoo really had no chance of having a "normal" life, but at least for the time he spent with the detective he had some sort of happiness. In the raw chapters we see the detective having flashbacks to all the life stages he spent with Sealwoo.
Kind of hard to lose whenever you have the fucking devil on your side lmao cop bit off more than he can chew