In the letter on Friday, addressed to Zuma, Kunene said: "In public you smile and laugh, but in truth you behave like a monster, a tyrant who will target perceived enemies ruthlessly, and because of that fear few dare to speak openly."
Kunene wrote that he supported Zuma before he ascended to the presidency, but was now disillusioned with his leadership.
"I'm writing this because I've never been more disappointed with the ANC you lead … Mr President, I don't want to be one of those who tell you in fear that you have clothes on, when it's obvious you are completely exposed," Kunene said, referencing the fairy tale The Emperor's New Clothes.
He said Zuma's relationship with the Gupta family and the recent wedding scandal, and controversy surrounding spending on Zuma's lavish household at Nkandla, detracted from the president's credibility.
"The Gupta issue alone should be the last straw for many South Africans," Kunene wrote.
"But the extent of how much the Gupta family controls you, and by implication this country, has not even begun to be understood."
He said many people within the ANC were "terrified" to speak out against Zuma because they feared him.
"After the Shaiks and Julius Malema, the Guptas must know that you can drop them faster than they could drop your name. In your quest for self-preservation, you have become heartless."
Kunene defended his own lavish lifestyle, saying that the money he spent was not taxpayers' money.
"I love the ANC, or what it's supposed to be, but I don't love your ANC," he wrote.
"For those of us who care, the question now is, as [communist theorist] Vladimir Lenin asked: 'What is to be done?'," Kunene concluded. – Sapa