King K Rool was an absolute dictator who seized control of Kong Island after the brutal assassination of the original King Kong at the hands of a simple plumber left an immense power vacuum. In light of a dead-locked parliament whose only uniting factor was a loyalty to the Kong family, Rool and his loyal followers, collectively known as the Kremling Krew, went on their famous March on Kongopolis (in which Rool himself technically didn't even participate in) to force the Prime Minister to dissolve parliament after recognising their sovereignity.
Unfortunately for him, the Kong family managed to escape prosecution by fleeing to the Southern Archipelago where King Kong's son, Kong II., managed to gather supporters for his cause, which eventually led to his establishment of a reactionary government aiming to re-establish his family's rule in the whole Kingdom.

Still facing internal problems from their takeover, the Kremling-controlled part of the kingdom did not have the military or industrial capabilities to oust the Kong rebels entrenched in the jungle forests, supported by the local population and large financed by exports of their main resource, the delivious Viet banana, to their supporters, primarily the neo-absolutistic Mushroom Kingdom.
The Rool government thus saw their only chance at controlling the rebellion (due to their almost exclusively banana-financed nature often referred to as the Viet-Kong) in putting a strict ban on banana exports in effect, something that was much easier with their naval superiority thanks to the support of the Klaptraps.

For a long time it has been considered consensus amongst historians that this prolonged civil war with constant small Viet-Kong guerilla attacks on vital industrial targets basically made King K Rool go mad. But this interpretation has recently been challenged.
Having suddenly skyrocketed from small opposition party to a position of absolute power there was obviously a certain amount of "mad with power" going around but for the most part his policies, although leftist-fascist in nature, were not all that terrible and many historians can now agree that the export restrictions, while very damaging for the banana producers and in extent local economy, were ultimately the most reasonable solution when compared to the possibilities of large-scale aerial bombardment or costly military ground operations.

It seems the commonly known image of King K Rool was mostly influenced by the anti-Kremling propaganda put out by the politically influential Kong family and to a certain extent the Mushroom Kingdom, whose ruler, the Mushroom King, lived in constant fear that once Rool had control he would be providing support to his political brothers, the Koopa rebellion under Bowser.