I remember that the main character is trying to find the group that destroyed his home planet while he was on a mission. His planet was very difficult to live on and the people became expert in martial arts and were the best soldiers in the galaxy. They were mercenaries and often helped against tyrants. There was a very memorable section in at least one book about an initiation task that the main character had to do in that he had to survive in their wilderness for days. He was at some point surrounded in a old building by thousands of poisonous worm/snake creatures that he managed to survive until rescued by the leaders.
The Last Legionary series (Galactic Warlord, Deathwing Over Veynaa, Day of the Starwind, Planet of the Warlord, Young Legionary) by Douglas Hill.
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.