Mills has the life and thinking of an everyday man; he is an eternal optimist. Somerset is cynical due to his experience, while John Doe (and I am not going to brand him as crazy like most optimists do) is a radical/extremist, maybe due to all the bad stuff in his life caused by his envy. In spite of his criticism on people's banality, he envies those people who can lead a normal life because he doesn't have one. He didn't change any of his plan, he only fast tracked it. Somerset can understand John Doe, not because he is a voracious reader, but because he is also disillusioned with life. Their difference is, Somerset accepts what's wrong in the world, John Doe couldn't. His envy drives him to kill because he wants to correct the things he sees as unfair.