>>718388937
Genestealer cultist are actually not wholly incoporated into the tyranid hivemind, instead they have a sort of "broodmind" which is a lesser form of tyranid hive mind bs, that gets subsumed by the proper hivemind once a hivefleet gets close enough.
Also, the hivemind of the tyranids doesn't prohibit individuality, hive tyrants, tyranid warriors, lictors and genestealers can and do operate as sort of "individuals" and could be described as having their own personalities, it is just that those personalities are tied to the overall hivemind and are slaved to it. They have a degree of autonomy but they cannot operate or go against the overall will of the hivemind. Hive Tyrants and Tyranid Warriors are also as intelligent if not more intelligent than most sapient races within the galaxy are, they are not mere animals like gaunts etc. For genestealers especially, this individuality and intelligence emerges in a more pronounced fashion once they are far enough away from a hive fleet as them being able to operate as semi sapient, cunning inviduals suits their role as infiltrators.
The tyranid hive mind should not be understood as a sort of "borg" style thing where every entity of the race shares one mind, it is more like a literal "hive of minds" that are all linked together, from the lowest form of tyranid microbe all the way to the biggest hive ship. They are not one mind spread across a countless number of bodies, they are countless number of minds of variying levels of sapience, all linked together via their synaptic hive mind, and bound to the same overarching goal of consuming biomass across the cosmos.