Lore needs to use the Crystalline Entity to kill the colonists as he's unable to disobey his creator; Dr. Noonian Soong.
We know that Lore was built with a more "human-like" personality than Data as well as being exposed to the scanned memories of the colonists on Omicron Theta. Whether this process is what pushed him over the edge of sanity or whether he was always going to go insane is not explained but regardless of the cause, Dr Soong identifies that the colonists were terrified of Lore and petitioned Soong to deactivate him, something that he was able to do with ease.
Soong stands and turns to Lore.
SOONG : You were the first. You meant
as much to me as Data ever did. But you were unstable. The colonists
weren't envious of you, they were afraid of you. You were unstable.
There's no canon reference as to when Lore first learned to communicate with the Crystalline entity but what's apparent is that somewhere between the date of his creation and his ultimate deactivation, he made contact with entity and drew it to the colony with the main intention of killing the colonists, something that Lore can't do because of the block on his free will
SOONG : (continuing to Data) Lore is far from the maniacal android you have made him out to be. In any case, he'll obey me. He
The Crystal Entity appears to be both sentient and intelligent, responding to direct verbal commands given by Lore over a communication channel. Since the entity isn't bound by the same restrictions as Lore (e.g. to obey Soong) it makes perfect sense that Lore would have contracted out the murders to a third-party hitman.