Is expiation for unintentional killing obligatory for someone who sprayed an insecticide that caused the death of people from lung cancer after inhaling it?

The default ruling is that it is permissible to spray insecticide in the home to eliminate the harm caused by insects. If someone does this without transgression or negligence, and harm results, there is no liability upon them. However, spraying insecticide in a room where people are present, while knowing its harm to them, is an impermissible act, based on the Prophet's (peace be upon him) saying: "There should be no harm nor reciprocating harm." If their action causes injury to them, then they are liable for what resulted from their action if they were transgressing, whether intentionally or not. The person who sprays the insecticide is not liable unless it is known with certainty that the injury was caused by their transgression, because the default is the exoneration of liability.