Yuval Diskin, who retired as Shin Bet chief in 2011, last year warned Netanyahu sought a "messianic" war with Iran.
"At play inside Netanyahu, in my opinion, is a mix of ideology, a deep sense that he is a prince of a 'royal family' from the Jerusalem elite, alongside insecurity and a deep fear of taking responsibility," Mr Diskin said in newspaper Yedioth Ahronoth.
Ridiculing Mr Netanyahu's "zig-zag" style in decision-making, Mr Diskin said the Prime Minister "has no strong core, no tough kernel about which you can say, 'Know what? In an extreme situation, in a crisis situation, I can follow him. I can trust him'."
Yedioth, which has often editorialised against Mr Netanyahu, published a response by the Prime Minister's office dismissing Mr Diskin's remarks as "fatuous".
Mr Netanyahu is still strongly favoured to win a third term.
Other retired Israeli national security figures have joined Mr Diskin in criticising the Netanyahu Government. At least one of them, former top general Gabi Ashkenazi, had his tenure in uniform cut short in a spat with Defence Minister Ehud Barak.