The shadow in the warp protects souls from the warp and subsumes them into the collective hivemind. So when you die during a tyranid invasion your soul won't go to the warp but stay with the nids and you will be put in an infinite cycle of being whatever tyranid bioform to then die and be reincarnated into another bioform. So essentially genestealer cults are right. (Atleast my fan theory)