A Brand New Theory!!!!!
Hmmm......
I had kind of a Eureka moment while watching this that might have been knocked down (or maybe not!) by the twist at the end.
I considered that everyone was killing everyone else - it wasn't the Judge killing everyone though he did kill the Doctor.
The Prince is killed by the housemaid Ethel.
Ethel is killed by the General.
The General is killed by the butler.
The butler is killed by Emily Brent.
Emily Brent is killed by The Doctor.
The Doctor is killed by the Judge BUT the Judge makes it appear that the detective killed him, so...
The detective is killed by Mr Lombard.
Mr. Lombard was supposed to be killed by Vera (she had the gun) but Lombard broke the chain and faked it.
Finally, the Judge was going to kill himself and leave Vera to die at the hand of the authorities because she'd be the only one left or have Vera kill himself.
Basically, the killer is in turn killed by a person who suspects him (and has a motive) - and in order to keep the idea that there's someone else in the house (to avert suspicion) they each in turn break an Indian statue.
It does appear (though I'd have to rewatch it) that each killer had in turn a motive for killing the killer of the previous victim. For instance, I can see the General killing Ethel (the wife) because of his past history (the story of his wife and lover). The butler would have killed the General to avenge his wife's death. Emily's been watching everyone at this time - she knows for instance that Vera did not kill the General because she had an opportunity and didn't use it. Emily suspects (knows) that the butler killed the General and so kills him, and so on...
The Judge knew that this would happen because he knows that they have all killed in the past so he just sits back and watches everyone kill everyone.