It seems so easy to ignore some simple things when one forms an opinion.
1. The talwars have reportedly said that they used to lock her room and on that day, they forgot to do so. Sometimes we too forget things like locking a window or closing a door. No one will keep worrying that the servant could kill or rape your dear daughter. If that was the case, they would have thrown out that servant. After sometime, there is a bound to be some amount of trust with someone who lives 24 hours in your house. Otherwise, it is difficult to have a servant living in. But, this is what a criminal mind will like to exploit. One mistake. That is all they need as an opportunity to activate their plans.
2. The servant must have invited others, and they started drinking after the parents went to sleep. Remember their room was closed from inside (which is normal for a couple to do!), and possibly bolted from outside (by those who killed) during the time of crime.
3. The role of Hemraj is difficult to guess at this stage. He may have opposed his partners thoughts of rape of Arushi. So, they may have killed him first (two (or three?) men overpowering a man, without giving a clue that he is going to be killed, is not that difficult), Remember there were three glasses of alcohol, and Hemraj did not drink as per the post mortem report on him. So, possibly, there were three men other than Hemraj inside the house.
4. If they were able to enter Talwar's room, they could have killed them.
