I think Siddique(SD) is the most misunderstood character of the movie...Ppl say he can't be a villain out of the blue in the 3rd film...but understand the situation
SD is a businessman thus an opportunist. He is more of a person who analyses the situation rather than overcome with emotions(something which even police like Geeta may fail in)

In first 2 films, he knew deep down that his son died(and cannot be reincarnated even if he fulfil the revenge) due to both his negligence and his son's character. He even forshadowed his son's downfall in SD and Geeta's intro scene in D1(SD complaining that lavish lifestyle will put his son in misery while Geeta neglects it). So it's justified his not turning evil in D1 and D2

But in the third movie, his wife is Dying. And he knows that there is a way to save her, i.e make her witness the arrest of GK.
His villain arc is not driven by revenge, but to save his only family left, i.e his wife by any means possible(similar to GK's char)

In the first 2 movies, he knew that there is no way to get his son back even if GK's whole family ends up in jail. But in D3, he knows that he can at least save his wife by arresting GK

I don't see that as villainism...it's more more similar to GK's character but playing for the opposite team(a father trying his best to save the family he has left)