The incident happened Monday evening when Galmit Sangma, 25, entered the house of Barning Marak in New Rongara and attacked the mother and child, the police said.
"Galmit Sangma attacked the duo with the axe leading to the immediate death of the child. The mother suffered grievous injuries as well," said SGH police chief, Abraham T Sangma.
Hearing the cries of the mother and the child, the villagers caught hold of Galmit Sangma and beat him to death, police said.
"He (Galmit Sangma) had been confined by his family due to the nature of his illness but managed to escape confinement yesterday. He immediately ran away and entered the house of Marak and attacked the infant and her mother, the police said.