"They are coming! I feel them scratching inside my mind, scratching, screaming, running, so many - so, so many voices. They are coming for us - flesh, body and soul!" Tyranid codex 4th ed.
This quote got me thinking. What if the Tyranids are consuming the galaxy's life with the best of intentions? If the Tyranids can consume souls (being a psychic race, it is not out of the question) perhaps the Hve Mind thinks that by making all life part of itself, it is doing sentient races a favor. It could be that the Hive Mind thinks that being part of the Hive Mind is pretty awesome and it wants to share this awesomeness with everyone. Those silly sentients who resist are just ignorant and will thank the Hive Mind later when they to are part of the Hive Mind.
The really crazy thing about this line of thinking is that when you consider what probably happens to humans who die and what definitely happens to eldar who die without a soulstone, the Hive Mind might have a point.