In the long run, the most vicious predators, like most dread disease-causing microbes, bring about their own ruin by killing their victims. Restrained predation—the attack that doesn't kill or does kill only slowly—is a recurring theme in evolution. The predatory precursors of mitochondria invaded and exploited their hosts, but the prey resisted. Forced to be content with an expendable part of the prey (its waste)... some mitochondria precursors grew but never killed their providers.... The original prey was probably a larger bacterium like Thermoplasma.
Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors (1986)