Apoptosis is largely regulated by mitochondria

Power, Sex, Suicide by Nick Lane (2006) part 5 "Murder or suicide" mentions that the key events that leads to apoptosis is when certain proteints normally present in the inner mitochondrial membrane spill out, and that this often happens when free radicals are produced in excess: the cell is really not doing well in those cases. This point suggests that the initial mitochondrial endosymbiosis happened due to a parasite that lived inside another cell. It mentions that even today we see parasites kill the host cell when they feel that the cell does not have many nutrients. This frees the parasites to then infect other cells.