Mr Loomis is an insane character in the novel "Z for Zachariah. However, he may be labelled as "evil", simply because his plans appear to be evil, although they actually may not. There are several instances/sources in the novel that suggests this, through his interactions with the other people who live around him. .
Ann Burden, a fifteen-year-old girl, is quite the contrary. She is essentially good and caring, with many instances that prove this. These two people are the main characters in the novel and it is exceptional to find how much their traits differ.
John R. Loomis is a chemist from Ithaca, New York in the novel. He is a man who has been through a terrible tragedy. He managed to survive a massive nuclear holocaust, which virtually wiped out the whole world's inhabitants. He survived initially in the underground laboratory and because of human inventions, such as the safe suit and the plastic "polapoly". There is a possibility that the effects of the disaster may have affected his mental state somewhat, possibly through the near-fatal illness he suffered (radiation poisoning) after the disaster.
There are arguments for and against Mr Loomis being evil. This strange man, nevertheless, is very intelligent and cunning. His devious plans prove how good he is at thinking strategically and logically, for his future and current situations. So here it seems he is not evil, he is just clever. Wanting to control the valley is not an evil mind-set, that must be part of one of his plans. When he kills Edward over the safe-suit altercation this is pure evil and insanity. He guarded the suit so intensely that there is a sense of complete paranoia. The same applies to when he shoots Ann in the leg, after she attempted to enter the store in the valley. Once again, however, his intelligence comes into the frame; he is deliberately trying to keep her out because if her leg becomes immovable, he then can control everything.