A red card for Joel Obi and a dubious penalty decision in the 43rd minute helped Walter Mazzarri's side on their way to all three points as Claudio Ranieri saw his side lose for the first time since he took charge.
Marek Hamsik missed the spot-kick but Hugo Campagnaro scored the rebound and further goals from Christian Maggio and Hamsik in the second half earned Napoli all three points.
The result took them two points clear at the top and left Inter just one point above the bottom three.
The ex-Chelsea manager was fuming after the game about being sent off for his protestations, and according to reports from Tuttomercatoweb, slammed: "[Gianluca] Rocchi was not good enough tonight, as Obi's first yellow was non-existent and the second one was outside the box".
"He ruined a potentially fascinating match and also our first-half performance."
The Italian tried not to make a fuss though, and gave commiserations to his men for their efforts.
"Complaining is not my forte, and I have only done so a few times in the past in 20 years, but I have enough experience to know whether a referee is having a good or bad day," said Ranieri.
"The boys did not deserve to be punished in this way. I was sent off because I complained too much, but we are men and it is normal to be frustrated when you are penalised by the referee", he concluded.