Its a common misconception that the Necrons are basically just robots that run around killing everything. In fact, the Lords, Pariahs, and Immortals to a lesser extent retain their sentience and consciousness and are fully capable of acting independently and interacting with intelligent species.

Another conveniently ignored fact is the plethora of evidence and examples of Necrons communicating with and even conducting safe passage for humans as guests or envoys. The C’Tan believe that humans represent their future in this galaxy and see us simultaneously as their new favoured servants, worshippers, and livestock. In their view their domination and our subsurvience is inevitable and they in no hurry to enslave, harvest, or exterminate humanity. They have all the time in the universe.

The Tyranids, on the other hand, represent an enormous threat the Necrons, even greater than Chaos. If Chaos emerges triumphant and the galaxy again unravels in warp catastrophe, the C’Tan can simply return to their tombs and wait another 500 million years and start over. If the Tyranids succeed in overrunning the galaxy, however, they won’t leave behind a single biological cell. There would be nothing left and the C’Tan would rot, wither and presumably they would eventually die out.

The Necrons have aided the Imperium before, in the fight against Chaos during the 13th Black Crusade, and it makes sense that against a foe like the Tyranids, they would do so again. They don’t care one way or the other about a single Blood Angels strike force. Humanity will serve or die in their own time, but a threat like the Tyranids cannot be ignored.

As for the Blood Angels… well, for all the anti xenos rhetoric in the Imperium, humanity has allied with virtually every major alien race at one point or another. And for all the talk of not wanting to battle the Necrons that he had just fought alongside, its probably important to remember that Dante was fully aware that he couldn’t actually beat them. Dante is not a stubborn fool and I’m sure he was more than ready to take the opportunity to save his brothers rather than drive them into a futile meatgrinder in the name of hate.