Esta é quase certamente a série Last Legionary de Douglas Hill. De Resumo da Wikipedia :
The books tell of the adventures of Keill Randor, the last survivor of his planet's population, who are annihilated at the beginning of the book Galactic Warlord. Randor's people were hardened over generations by their inhospitable planet, which (combined with rigorous combat and martial arts training) resulted in them exhibiting speed, reflexes, strength, and fighting abilities all at the very upper limit of human ability and Randor himself is one of the most skilled amongst them, twice winner of the planetwide martial arts contest, a feat bettered by only one other legionary in the history of the contest. The people of Moros acted effectively as mercenaries; however they were careful to only offer their services to clients fighting in self-defense. After they are all killed by a mysterious villain known only as the Warlord. Randor vows to avenge them.