Anarchaos — Donald E. Westlake

An early work by one of Science Fiction's grand masters
Anarchaos is a planet, inhabited by humans, where anarchy is the only law; where each man protects himself as best he can; and where the weak are soon dead. Malone's brother had dies that way, and Malone has come to Anarchaos, carrying a small arsenal of weapons, to find the man who killed him, knowing that he is facing an entire planet of enemies.