It propagates copies of itself by destroying all the other genes in the individual. So individuals, like social insect colonies, are normally unified adapted units, but there are instances where that unanimity breaks down. When you think of it, a individual like you or me, is historically a group, in several senses. This gives it a within-individual advantage that would cause it to spread But only temporarily. In the next generation, the progeny begin as a single cell, either selfish or non-selfish, but it can't be both.
Eukaryotic cells are themselves a group of formerly independent organisms.
This cooperative arrangement was selected for other reasons, not kinship. Another kind of false altruism. What was good for one was generally good for the other if they all had to reproduce through the same cells. If instead, the initial mitochondria could reproduce horizontally and move from one host to another, it might be advantageous to use up the host cell's food, burst out of the host and go find another to infect. Pathogens like viruses do this.