Killing the kid girl made a mockery of the plot and showed the intellectual bankruptcy of Story/ Script writer/ Director. She was utterly innocent and uninvolved. She couldn't have been blamed for whatever his father might have done. And the entire summary of the plot had depended on the mastermind being wrongly jailed on the basis of false proofs, so he was having an aura of taking revenge. But when the same mastermind kills a kid girl who was even more innocent (the mastermind had killed his own father, the kid girl didn't do anything wrong), then that aura disappears and no viewer sympathizes with whatever was done to him in childhood, viewers rather wish that the mastermind should have been punished even harsher, even killed. Now, all viewers want to see the mastermind nabbed and punished. The murder of the kid girl justified every wrong that was done by whoever to the mastermind. So, the entire emotional fine balance of the plot was thrown to winds by that one act, that was totally unrelated and could have been easily avoided.