Algeria international Faouzi Ghoulam led the charge for Napoli's opening goal as Dries Mertens tucked home his effort in the 34th minute.
After the restart, Benevento pushed for an equalizer, forcing Koulibaly to receive a yellow card as he tried to thwart the attack of the Sorcerers.
Matteo Politano doubled the Parthenopeans’ lead in the 66th minute, pouncing on a loose ball before firing his effort into the back of the net from close range.
With nine minutes before the end of the game, Koulibaly was shown his second yellow card and was subsequently sent off after a reckless challenge on Gaetano Letizia.