Pseudogenes are junk DNA. They no longer have a function. Perhaps the killswitch could be in a RuBisCo-type enzyme. RuBisCo is a plant enzyme that fixes carbon dioxide to make glucose. It also fixes oxygen, however, which is hypothesized to be a remnant from the early atmosphere where oxygen was rare. Oxygen fixation by RuBisCo, or photorespiration, results in ammonia, which the plant must use energy to detoxify.
My idea: an enzyme based on RuBisCo, which is part of an essential boichemical pathway, but causes problems in the presence of a specific substance. This substance would be manmade and very rare, and the effects on the organism would be fatal.