The hybrids created partly from Alien DNA and from that of the abducted daemons proved aggressive and unpredictable, creating problems at every turn, for the alien scientists. Who hoped to duplicate and farm the resistance of the daemon gene and eventually breed out a deficiency in their own gene line.
But the Alien Grey's inability to control these new creatures, resulted in a decision being made to terminate all remaining test subjects of the daemon-alien hybrid project, until such a time that security could be guaranteed.
It was always assumed that the mass cull was completed successfully and the last hybrids were killed upon that last evacuated ship, but a usually reliable source suggests otherwise.
Madok Erl, sometime assistant to Hobbs, refers to this event, in 'primus daemon', the definitive Daemon history text.