Why must androids on Earth be killed ('retired') - what is the nature/degree of threat they present?

Androids are considered anathema to life on planet earth because they are not real and because they do nothing to further humanity’s existence. In this future world, all life must have a purpose, either to care for or to be cared by, but androids are not able to do either. They can only be concerned with their own survival. This causes them, in turn, to take other life when given no other choice.