A Murder is also a name for a group of crows.
The Planet Murder
As seen in the first HH book, Horus Rising, Murder is a planet encountered by the expedition fleet commanded by Horus Lupercal himself. Initially, the Blood Angels made planet-fall with an entire company, but were completely annihilated by the natives (who turned out to be lightning fast bugs the size of elephants with claws that could RIP AND TEAR through power armor with ease). Fortunately, they succeeded in sending out a distress signal which the Emperor's Children (commanded by a certain sick fuck) responded to. When the natives wiped out half a company of said Emperor's Children, they too turned the wheels around and sent out their OWN distress beacon.
And this is where the Luna Wolves came in. They found a ring of abandoned space stations surrounding the planet, all of which were broadcasting a strange signal composed entirely of heavenly singing. In addition, there were also multiple ships reduced to various types of shredded debris in orbit. The Sons of Horus discovered the distress beacon of the Emperor's Children, and sent down a PROPER invasion force to rescue them. With the pink-sissies secured, Horus ordered the complete destruction of Murder, and for the Megarachnids of that world to be purged to the last.