i think Avni's killer is Divya, whose character is shown positive but mysterious. She might have gone to Avni's room to tell her not to come b/w Anjali and Aman. Avni could have gone mad and took out her gun. They start fighting and accidently, divya kills Avni.
Another possibility (but the least possibility) can be that Divya kills Avni on purpose so that Aman's child is killed. She knew that Anjali can't bear the child. She wanted her to-be-born baby to get the property.
Anyways, what do you think?