The deviant's role in the movie is kind of a twist. First, they acted as the reason for the appearance of the eternals. then it turned out they were just a kind of distraction. there was no typical real hero x villian, black x white in this movie. there was only conflicts based on differences in ideals, morals and views on life. deviant was now the 3rd party to further emphasize the climax conflicts. One side just wanted to follow orders, and do the same as what they al already did for years, accepted the destruction of the earth so that new celestial beings could be born, continue to create, research and develop new creatures in the future. the other side did not want to abadon the earth, wanted to protect the people whom they have spent long time with. Meanwhile, now deviants were like pitiful creatures that just wanted to continue to survive (by feeding on people) and acted based on survival instincts (probably deviant wasn't smart enough to understand the real upcoming situation. Maybe only Kro knew so he wanted to steal the Eternals's power to evolve with the hope to escape).