Media reports say the pair were moving through thick bushes near the town of Postiglione in the southern province of Salerno when the shooting occurred.
The 34-year-old opened fire when he saw a shadow and rustling foliage. He immediately raised the alarm when he realised what had happened.
Doctors could do nothing to save 55-year-old Martino Gaudioso.
Both men were in a national park area where hunting is prohibited, reports said. Police have seized their rifles.
On Sunday the president of the Italian League for the Defence of Animals and the Environment said Italy had become the "Wild West".
"It is a real national emergency," Michela Vittoria Brambilla told reporters.
Last October, Sergio Costa, Italy's environment minister, called for a national ban on Sunday hunting after an 18-year-old was shot and killed close to the French border.
By the end of that month, two more men - a 56-year-old and a 20-year-old - had also died in similar circumstances.