So it's 515 BC in the state of Wu, things were not looking very stable since prince Guang had decided it's time to assassinate the king.

That scoundrel, his cousin - King Liao had sit on the throne long enough, prince Guang bitterly thought as he recall years back, when his father passed the throne to his uncle instead of him. And now the chance was at hand. Firstly, Guang convinced king Liao to attack their rival state of Chu, directly weaken the military around the capital so he can even attempt a violent take over should the plan go south. The Wu army, lead by 2 of King Liao's sons no less faced difficulties and at risk of being cut off and destroyed. Prince Guang rejoyed at his nation's peril, such a perfect time for the coup. He invited the king to his mansion for a lavish feast, where his henchmen Zhuan Zhu dressed as a servant, would carry out the act. At that party, the king's bodyguards did not alert when this unassuming servant approach with a big fish dish for the king table, they pat him down and let him through. That's all it take. Zhuan pulled out a dagger from the fish and stab the king to death, the assassin himself soon too died under the blade of the bodyguards. But what's done is done, prince Guang became king Helu, the 24th monarch of Wu.
His trouble has just began however, as Liao's son - prince Qing Ji has escaped to another state where he would rally his supporters and build his own army for vengeance. What a headache, Helu cannot just march an army there to kill Qing Ji without risking everything. How about sending another assassin? It worked before.

Entered Yao Li, a strong and fiercely loyal man. Helu tasked Yao Li with this monumental affair: to approach prince Qing Ji, gain his trust, and kill him. Just to give Yao more credibility, Helu would "punish" the former harshly for a made-up crime. By harshly I mean cutting off his arm and execute his family. There was no reward either, Yao Li was not expected to return alive. And yet Yao agreed. With one arm missing and the whole family dead, he came to the exiled prince and asked if he can join this quest for vengeance. Qing Ji welcomed him with open arms. There is no chance in hell someone would be a spy, much less an assassin working for the king that had treat him so horribly. In fact Yao soon became one of the prince's most trusted companions. One day, they ride a boat to inspect the river together, Yao use his spear to impale Qing Ji, all through his master's body.

Prince Qing Ji, with spear still in his stomach, overpowered Yao Li. He grabbed the one-arm assassin by the neck, press his head into water three time before throwing him out to the boat, laugh and tell his guards: "Don't kill him, there should not be two brave men to die in the same day. Let him return to Wu and be rewarded for his loyalty." All the time, he just seem almost amused that Yao Li would go that far to kill him. Qing Ji died of the wound and Yao Li was set free.

The story did not end happily for Yao Li either, why would it? He condemned his own action as inhumane as he had agreed to let his own family killed, unrighteous as he has help the new king assassinate the former king's son, and finally blatantly stupid as he has sacrificed everything for what? Ridden with guilt and regret, Yao committed suicide afterward.

Helu, on the other hand, was having the time of his life. With the help of able ministers like Wu Zici and allegedly a certain "Sun Tzu", he built a strong nation and defeated his rival state of Chu multiple times. But heaven is ever unpredictable, poetry justice will find him soon enough.

Source: this story is from Chronicle of Eastern Zhou and "Spring and Autumn annal of Wu-Yue", both are classic historical novel. You could probably find a translation somewhere on the internet.