In what was believed to be his first interview for television, Prachanda told the BBC on Monday: "The king has taken steps that do not give any room for compromise. It would be correct to say that the path that he has taken is the road to hell."
King Gyanendra seized absolute power in Nepal just over a year ago.
Prachanda, who has lived in hiding for more than 20 years, forged a loose alliance with the country's main political parties last year, aiming to topple the king and restore democracy.
The Maoists began an uprising in the west of Nepal in 1996 to overthrow the world's only Hindu monarchy and install communist rule.
In the subsequent years of conflict, at least 12,500 people have been killed and the economy, heavily reliant on tourism, has been left in ruins.
Prachanda said in the interview: "I believe that it [Nepal] will be a republic state in less than five years. The king, I think, will either be executed by the people's court or he might be exiled. For the king, today's Nepal has no future. We don't see a future for him and the Nepali people don't either."
During the interview, filmed in an undisclosed location, Prachanda also blamed foreign interference for his group's failure to take the capital, Kathmandu, by force.
"When countries like the US the UK and India started supporting the royal army militarily against our people's war and the revolt of the Nepali people, that ... posed some difficulty," he said.
'New plane' of leadership
"That is why we believe that in today's world it's not possible only to move forward militarily. Today's reality is to move forward both politically and militarily, with a balance of the two."
"It would be correct to say that the path that he has taken is the road to hell"
Prachanda, Maoist leader
Some observers have suggested that the Maoists' recent shift towards multi-party democracy is simply a cynical ploy, but Prachanda denied that he had aspirations to become head of state himself.
"If need be, and if necessary for the Nepali people, I am of course ready for it," he said. "But I also want to clarify that from the lessons of the 20th-century communist states we want to move to a new plane in terms of leadership where one person doesn't remain the party leader or the head of state."