I'll cite a bit of A HISTORY OF DEMOCRATIC KAMPUCHEA (1975-1979, as produced by the Documentation Center of Cambodia (DC-Cam). It is a great primer to Democratic Kampuchea (the state under Pol Pot and the Khmer Rouge) and its bizarre, grim and sad history.

The Khmer Rouge also began to implement their radical Maoist and Marxist-Leninist transformation program at this time. They wanted to transform Cambodia into a rural, classless society in which there were no rich people, no poor people, and no exploitation. To accomplish this, they abolished money, free markets, normal schooling, private property, foreign clothing styles, religious practices, and traditional Khmer culture. Public schools, pagodas, mosques, churches, universities, shops and government buildings were shut or turned into prisons, stables, reeducation camps and granaries. There was no public or private transportation, no private property, and no non-revolutionary entertainment. Leisure activities were severely restricted. People throughout the country, including the leaders of the CPK, had to wear black costumes, which were their traditional revolutionary clothes.

Their ideas were indeed radical and seeked to uproot society: unlike its neighbors' processes of import substitution and then foreign exports as a policy to build up an industrial base (most capitalist neighbors, such as Malaysia and Singapore) or even a moved to centrally planned industrialization (Vietnam), the Khmer seeked to establish an agarian society: the 'rural, classless' state mentioned above.

You do note that their reference to Maoist theories seems irrational and at odds with the goal of governance: forced labor and deportation to the countryside, targeted killing of social undesirables. However, rather than not being the 'initial idea they had', it was an extremely targeted policy beginning from their initial occupation of the very first cities liberated/occupied in the wake of the civil war.

As Time correspondent David Aikman notes:

Then came the shock. After a few hours, the black-uniformed troops began firing into the air. It was a signal for Phnom Penh's entire population, swollen by refugees to some 3 million, to abandon the city. Young and old, the well and the sick, businessmen and beggars, were all ordered at gunpoint onto the streets and highways leading into the countryside. ...The survivors were settled in villages and agricultural communes all around Cambodia and were put to work for frantic 16-or 17-hour days, planting rice and building an enormous new irrigation system."

The forced depopulation of cities was part of a deliberate programme to resettle them: those moving from cities into the countryside would be classified as "new people", who would be contrasted against those already dwelling in the countryside: the "base people". Going back to the Dc-Cam document:

They were classified as “parasites” and had no rights as the Khmer Rouge slogans asserted: “17 April people are parasitic plants. They are the losers of the war and prisoners of war.” Another slogan, recalled by many survivors, was: “To keep you is no gain; to lose you is no loss.”

The mass killings therefore targeted Cambodians by class as well, primarily the intellectuals as your question has noted but also more broadly those that had fought against them in the civil war, political dissidents or even those called out by informers.

Killings were also targeted at racial minorities: Vietnamese and the Chinese amongst others, with the former ostensibly belonging to a hostile state and the latter considered part of the wealthy class of people. Vietnam would indeed invade them in 1979, deposing the Khmer Rouge. China's support, on the other hand, was unable to excuse the targeting of ethnic Chinese too, despite being one of its staunchest supporters in the region (doing so to serve as a counterweight against the Soviet-backed Vietnam). Now going to Elizabeth Becker's book When the War was Over:

Khmer Rouge policies that were either specifically directed towards minorities or had significant effects on them can be divided into four categories: i) imposition of uniformity, ii) expulsion, iii) extermination, and iv) other forms of discrimination.

The Khmer Rouge sought to impose uniformity on the population by using “forced Khmerization,” requiring minorities to abandon aspects of their distinct culture and to become “Khmer.”

Racial minorities were also targeted due to their perceived opposition to the state: an entity that was not just political in nature, but racial and racialized at the core. To this extent, undesirables would have to be purged.

Hope that answers your question about what they set up to achieve, and why killing figured so prominently in the equation and to such a tragic degree. Sources are all linked above. Have a good trip!