This was because the Killers did not engage in standard hunting practices but instead sought only to kill rather than hunt, which many Hunters found no honor in. The hunt is sacred to the Hunter Predators so this focus on killing was considered deplorable. This led to a conflict between the two groups; a war which was believed to have destroyed the rogue clan. Following that event, the story of the Killers became a myth amongst the normal clans.
However, the clan was not destroyed and continued to operate in space where they raided as well as destroyed any colonies they discovered. As part of their operations, they made use of Xenomorphs as hounds to hunt down their targets. One of their raids led to the destruction of a Human colony but not before footage of the incident was recorded by the military. They later approached Machiko Noguchi and after watching the video, she revealed the existence of the Killers to the marine forces.