Yes First of all, there is an insider in this case (see Chapter VI, paragraph 1 1, from "if you have a friend here" to "escape from the road" explained by Holmes). Later, it was mentioned that Jonathan came to London for a short time, because he was an entertainer and could not buy off the people in the villa, so the insider must have helped him because he knew him well. Jonathan stayed in India for a long time, but not in England. So the insiders should be Indians. Shulte Jr. mentioned that his father brought back some Indian servants from India, so the insiders must be among them. Only Rahl Rao was still alive at the time of the crime. Besides, the housekeeper mentioned his bad behavior.

Besides, other people in the villa can't be insiders, and killing people is no longer Jonathan's plan. They fled in a hurry, so the insider can't be the housekeeper who caused everyone to panic in the suburbs. It can't be that McMurdo knew the identity of Sherlock Holmes and let them in (McMurdo said that Sherlock Holmes is not a professional boxer, but it shows that he knows the identity of Sherlock Holmes). Williams is a native Englishman and a lightweight boxing champion. It is impossible to be pursued by Jonathan, who only earns a copper hat every day, not to mention that he was on his way to pick up Holmes when the crime was committed.

Therefore, there must be an insider in this case. The inside man is Rahl Rao. He is what Holmes called an accessory.

Note: I answered similar questions in Zhihu. The two answers are the same, both original. Hereby explain.

By the way, the one above mixed the story of uncanny valley into Four Signs. There is no hostess in this manor.