The Heart secretly kept track of all the rogue Droods, and murdered any female rogues.There are so many male rogue Droods seen or mentioned in the series, and so little reference to female rogues, that something must have been eliminating the latter systematically. The Heart altered the Droods' genetic code so they'd always produce twins, ensuring itself a steady supply of infant sacrifices. For male Droods, fear of being found by their family meant the rogues could be counted upon to keep moving and/or isolate themselves emotionally; therefore, the Heart didn't expect any of them to settle down with a woman long enough to notice they were only siring twins. A female rogue, however, couldn't possibly overlook it if she kept giving birth to twins, which might lead to her asking awkward questions about why it was happening and whether other Droods did the same. To guard its hidden agenda, the Heart covertly kept track of the rogues' torcs (without telling the family it had that ability), then de-activated any female rogues' armor via long-distance remote control at the first opportunity to get the wearers killed.