If someone knocked on the door, one would think that that pesky security camera would have caught them, but the neighbour said nothing about that. The reason I think a sophisticated hitman fits the profile is because the scene appears to be void of any evidence they were there. No evidence of them arriving or leaving. No evidence of forced entry. If a sophisticated killer was meticulous, it could be done. Some random acquaintance, who is committing his first murder, likely wouldn't have the wherewithal to leave no evidence of his presence. MOO.