The dramas are similar, a man's main year will have to decide between love and revenge. The role of Hua Jai Look Poochai is retaliating for the murder of family and friends, but in Hua Jai Sila, the male role takes revenge on the family who has trodden him in childhood because he was the adopted son of his father, a poorly born woman.
Both have gorgeous male leads, both are melodrama tear-jerkers, both have crime and action plots that are pretty solid, dark, and satisfying. The only major difference is HJS is more about the romantic relationship, and FFKD has a very strong estranged brothers and family plot. Both can get pretty dark. But imo the fl in HJS might be a pushover, I never resented her because being submissive is not wrong. But the fl in FFKD has some seriously problematic manipulative behavior.