By definition, man v. man involves two human beings as opposed to man v. himself or man v. nature. For example, a man's struggle with his own feelings would constitue man v. himself, a man's struggle with a jaguar or tornado would illustrate man v. nature, and a mental or physical struggle such as that between Rainsford and General Zaroff in The Most Dangerous Game is man v. man.
Man himself contains both good and evil. Man is neither purely good nor purely evil. Therefore a true, pure battle of good and evil cannot exist within the conflict of man v. man. However, one man may be clearly morally superior to the other, and the reader considers him "good" while the other "evil," but this is subjective. Most readers will agree that General Zaroff is "evil" and morally corrupt as he hunts humans for sport. Rainsford is likeable and the character we feel the need to cheer for, making him "good"--if only by default.