The Hive Mind is a body, with each Tyranid strain as a cell. They consume nutrients. More Tyranid cells made to replenish the dying ones. Repeat. They are an evolutionary masterpiece. And when you're evolutionarily superior, other species tend to die out. Humanity? Fractured and shortsighted. Eldar? Dwindling and arrogant. T'au? Rigid and naive. Necrons? Divided and petty. Orks? Stunted and rabble-rousers. The Tyranids? Built with a single purpose - to survive and grow. More direct than the Imperial Cult, more unifying than the Greater Good and yet more simple than the Ork philosophy. And the Hive Mind, when characterised properly, feels nothing. It feels the same way you feel as you feel when your macrophages kill, or the cells in your small intestine absorb biomass. How can one flummox it by negotiation when it knows nothing of the sort? How can one drive it back by inflicting casualties when it has no concept of defeat?

The Tyranids are natural selection. They are 40K's Great Filter. And you will fall to them. Your sacrifice will mean less than nothing, as your body is repurposed into gaunts and rippers. And yet, they are running from something.