Actually in the case of removing all oxygen atoms from Earth's gravity well, not necessarily. The AI might decide that the most expedient method is to persuade all the humans that the sun's about to go nova, construct some space elevators and Orion Heavy Lifters, pump the first few nines of ocean water up into orbit, freeze it into a thousand-mile-long hollow cigar with a fusion rocket on one end, load the colony ship with all the carbon-based life it can find, and point the nose at some nearby potentially-habitable star. Under this scenario, it would be indifferent to our actual prospects for survival, but gain enough advantage by our willing cooperation to justify the effort of constructing an evacuation plan that can stand up to scientific analysis, and a vehicle which can actually propel the oxygenated mass out to stellar escape velocity to keep it from landing back on the surface.