The prisoner escaped from the territorial prison. (Are we starting to see a trend here?) That prisoner teamed up with another individual, and hunted down Mahaska. He had been hunting and was camped along the Nodaway River. Then they murdered him.
Quoting directly from McKenney, Vol. II (1872):
“The tidings of Mahaskah’s death soon reached his village. One of the murderers escaped, and sought refuge among the Ottoes; but, on learning the cause of his visit to them, they shot him in their camp. The other, with the utmost indifference, returned to the village of the murdered chief. Young Mahaskah, now the successor of his father, and principal chief of the nation, on hearing the news of his father’s death, and that one of the murderers had returned to the village, went immediately to his lodge, killed his dogs and horses, and with his knife ripped his lodge in every possible direction. Having hurled this defiance at one of the murderers of his father, and expressed his contempt for him under every possible form, he turned to the assassin, who had observed in silence the destruction of his property, and, looking him sternly in the face, said–‘You have killed the greatest man who ever made a moccasin track on the Naudaway; you must, therefore, be yourself a great man, since the Great Spirit has given you the victory. To call you a dog would make my father less than a dog.’ The squaw of the murderer exclaimed to her husband, ‘Why don’t you kill the boy?’ He replied, ‘He is going to be a great brave; I cannot kill him.’ So saying, he handed the young chief a pipe, which he refused, saying, ‘I will leave you in the hands of the braves of my nation.’ To which the inflexible murder replied, ‘I am not going to run away; I’ll meet your braves to-morrow.’ The Indian knew full well the fate that awaited him. He felt that his life was forfeited, and meant to assure the young chief that he was ready to pay the penalty.
The next day a general council was convened. The case was submitted to it. The unanimous voice was, ‘He shall die.’ It was further decreed, that young Mahaskah should kill him; but he declined, saying, ‘I cannot kill so brave a man’ Whereupon he was shot by one of the principal braves. His body was left on the ground, to be devoured by wolves, as a mark of the disgust of the tribe, and of their abhorrence of the assassin of their chief.”