Honor killing is murdering a family member that has brought shame or dishonor upon the family, community or religion. Caste system is alive in India & across castes, a women's conduct is given very high importance in calculating family honor. Honor killing is prevalent in (Northern) India, where a girl from upper caste is murdered by her family for marrying, eloping and/or having sexual relations with a boy from lower caste.
Dhadak is an official remake (in Hindi) of a popular Marathi (language) movie Sairat. Sairat is a movie about honor killing where, a girl from upper caste elopes, marries & settles down with a child, with a boy from lower caste. The girls father sends henchmen to murder the girl & boy. Their child survives as he's not home when the henchmen arrive.
Dhadak is faithful to Sairat except at the end when the henchmen murder the boy & the child but leave the girl alive. Although all 3 of them are in the house for the henchmen to kill, they wait till the girl is out of the house & then kill the boy & their child.
Why did the father not want to honor kill his daughter? Especially because a prior scene shows that he loses an election to his rival due to said dishonor. His elected rival makes a jive at him to first take care of his daughter before thinking of the electorate.