He encountered the Pak Phssthpok who visited the system to discover a legendary lost colony of Pak breeders. Brennan realised that Humans might be descendants of those breeders and Phssthpok would annihilate them, following the Protector urge to sense and destroy mutations.
He used his superior intelligence to engineer social change in Earth's medical technology and social attitudes that eventually reduced the use of organ banks to reasonable levels. Part of Brennan's manipulation was the development of a science known as 'psychistry' used to 'correct' all forms of 'mental aberration'—the populace is incredibly docile.
He engineered an ice asteroid strike on the planet Mars, thus exterminating the Martian race. To Brennan as a Protector, it is self-evident that the Martians' continued existence is a potential threat to humanity.
Brennan calculated that the Home colony's destruction is necessary to create an army of childless human protectors to battle an invading Pak fleet and released a genetically modified Tree-of-Life virus to transform everyone between ages 40 and 60 into a protector, and killing the remaining millions of colonists. To Brennan, the deaths of innocent people are a logical trade-off to preserve the bulk of humanity and his descendants on Earth.
Yet Brennan is unable to kill his own breeder descendant, Roy Truesdale. Instead, Truesdale killed Brennan in an attempt to save the people of Home once he realizes Brennan's intention; however, after he is transformed into a protector, Truesdale personally completes Brennan's plan.