Complex sentence: Sherlock traces the Golem through his network of homeless persons, but is too late to stop the murder of another victim, an astronomy professor whom the guard had talked to, having realised that a recently discovered painting by Vermeer was a fake.
Split into simpler sentences: Sherlock traces the Golem through his network of homeless persons, but is too late to stop the murder of another victim, an astronomy professor whom the guard talked to. The guard had realised that a recently discovered painting by Vermeer was a fake.