It's possible that her father might have heard her begging for death from time to time and one day there was an accidental overdose which he blames HS for.
She doesn't seem like the type to commit suicide but there was something that happened which is linked to the insanity in their love that resulted in her death. From what HS said to Minne, I just felt that he would go beyond the usual for Imarti.
I understand what you all are saying about legalities, but that's the whole point here. He may have put those thoughts aside and only focused on what he thought Imarti needed. It's a question of moral dilemma and creative liberties aside, there is some ambiguity about the whole thing.
Did he not release Ashok on Babita's request, even though as a cop he was going against his principles?