the tyranids are a race of mindless killers whoo seek nothing other than destruction.
In 40K novels the Tyranids have been described as working solely from insatiable hunger with their intelligence only spiking when they find a particularly difficult species to eat.
Indeed, it is doubtful the Tyranid Hive Mind even comprehends such concepts. Rather, they invade to harvest valuable biomass and feed their insatiable hunger. The Tyranids require an endless supply of food, not only to nourish the hive fleets, but to grow new organisms.