Question: Why are the people being infected by the pesticide only seeing messages to kill from electronic devices? Why not from hearing voices in their head or from reading a newspaper, magazine, etc.?
Answer: Two reasons. One, it was The Beginning of the electronic age, when devices were becoming an everyday fixture in our lives. Second, to make sure the right message was sent to the right people to perform a specific task.
Answer: His fear of a conspiracy, that someone was control the entire experiment, like the Cigarette Smoking Man.The messages were part of his fear of being controlled. DO IT NOW. When he stopped the shooter in the tower, he knew the experiment was over, hence the message. ALL DONE.BYE.BYE. But it being the X-Files, was the last message real or an hallucination.