According to Gauri's flashbak, it was Samar who killed Nikhila.
Nikhila's last word was "Samar". Then on the hill Gauri was talling Samar to stop before the black car pushed her over, and then she saw Samar at the top of the hill.
So according to Gauri, Samar killed Nikhila, then convinced everyone that it was really her.
So, what do you think?
Did Samar really kill Nikhila, and blame it on Gauri?
If not, do you think there's a third person who did it?
Yuks