Though your assumption seems good, has lot of loopholes too. Why they had to hide Hemraj's body? If we go by your assumption, why did they hide Hemaraj's body? They could have left that as it is in the place where he was killed. They definitely would have had fear and tried to run away after committing crime. The time taken to kill both of them would have been few hours. There is no sign of any resistance by both the victims, that suggests both were drugged. How can someone enter a house and drug people? Killing a person in a small room like Arushi's would have generated a lot of random movement that would have disturbed everything in that room. Blood would have spilled all over including walls and roof. There is no sign of anything like that in that room - which is a strong indicator for assuming she was drugged. Why didn't killer secure themselves by locking Dr.and Mrs. Talwar in their bedroom. Killers have not moved out of the house with blood stained/soaked clothes and hands. They have cleaned themselves, collected all the pieces of evidence and move out in a relaxed manner. How can these events happen and both the parents are unaware of the same? At least of them is directly or indirectly involved.
