An own goal from Napoli goalkeeper Jose Reina cancelled out Dries Mertens' opener and earned Livorno a valuable point in their battle to avoid the drop.
The outcome in Leghorn followed a 1-1 result against Genoa in the previous round and saw Benitez's side miss another chance to close the gap on second-placed Roma, who are six points ahead of third-placed Napoli and have a game in hand.
"We had the chances and we should have killed the game off," the Spaniard said to www.sscnapoli.it. "We needed to win this game and I'm disappointed with the performance from all of my players."
Napoli were without their top scorer Gonzalo Higuain through suspension, but Benitez did not feel that should have been a factor.
"I don't think we depend a lot on Gonzalo," Benitez said. "We have so much quality up front to be able to always score.
"I think what was missing was the determination in order to score the winner.
"It's really a shame that we threw away two points because we had everything in our favour to take the win."
Napoli face a big hurdle next weekend when they host Roma at the Stadio San Paolo.
"We have to take it one game at a time," Benitez said. "There is no point in focusing on our position in the standings at this stage of the campaign. It wouldn't be useful.
"What we have to do is work and prepare in the best possible way for the game against Roma."