story identification - series of young adult science fiction books from early 1980s about last of a race of martial experts that sees the destruction of his home planet

The Last Legionary series (Galactic Warlord, Deathwing Over Veynaa, Day of the Starwind, Planet of the Warlord, Young Legionary) by Douglas Hill.

From 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 [...]. The people of Moros acted effectively as mercenaries; however they were careful to only offer their services to clients fighting in self-defense.