Mandela was an African revolutionary, a man who urged the previously non-violent (and totally ineffective) African National Congress of South Africa to begin "armed struggle" against the racist & semi-fascist apartheid government, because the ruling party had closed off all other options.
That alone led some conservatives to hate him as a "terrorist," as some are indicating here in YA Politics.
Mandela also worked with white and black South African Communists, since the South African Communist Party at the time had some of the bravest and most progressive white people opposed to apartheid as its members. No other white South African organization came close to it in supporting majority rule and democracy on a non-racial basis, so the ANC willingly cooperated with it.
In fact, Mandela himself at one time was a Communist Party member, although he let his membership lapse as he turned more in the direction of African radical nationalism.
At the time Mandela was heading the ANC, only to get caught, tried in court, and sentenced to life in prison, the US government gave lip service to opposing apartheid. But in fact the US government was quietly supporting the system, under "liberal" Democratic presidents (eg Carter, Johnson, Kennedy) as well as "conservative" Republicans ones.
Why? First, a number of nationally prominent US politicians were somewhat racist themselves, and inclined to sympahize with the badly outnumbered South African whites who had elected a brutally repressive and semi-dictatorial government to save themselves from rule by the non-white majority.
In addition, dozens of large, profitable US corporations (for example General Motors, IBM) were fairly heavily invested in South Africa. Why? Partly because of its wealth from diamond mines, gold mines, and other mineral wealth.
Partly for exactly the same reasons big US corporations invest in "Communist" China today -- despite supposedly hating Communism.
In today's China, as in apartheid South Africa during the 1970s and 1980s, the vast majority of the working people have few if any legal rights, and this guarantees that huge numbers of people are so poor & hungry they will work for very low wages for any big corporation that will hire them.
Investing in China today, or in South Africa in the racist and repressive 1970s and 1980s, thus amounts (amounted back then) to a sure route to HIGHER PROFITS for big US corporations.
Let's be real, okay? The US government, which tends to be dominated by big corporations & their lobbyists, wasn't going to put that cozy & profitable arrangement at risk by supporting a radical nationalist / and/ or leftist revolution against apartheid.
Just as the US government today -- representing those same big corporations -- does VIRTUALLY NOTHING to pressure China to stop occupying Tibet, for example, and does very little to pressure the Chinese government to stop repressing its labor unions.
So US corporate profits partly relied on a strong white apartheid state in South Africa. And the US government under LIBERAL DEMOCRATS AS WELL AS CONSERVATIVE REPUBLICANS therfore repeatedly rejected resolutions by the UN General Assembly, demanding that the world impose an economic boycott on South Africa to end apartheid without bloodshed.
In fact, BOTH liberal Democratic administrations & conservative Republican ones used the US VETO in the UN Security Council to make an economic boycott of South Africa impossible.
Mandela, plus the largely black & increasingly "revolutionary" African National Congress, plus the relatively few white South Africans -- both inside and outside of the Communist Party -- who supported the black majority struggle all threatened US corporate profits and US access to scarce raw materials.
And as noted, the ANC and its fighters were generally aligned with the international Communist movement, since most western capitalist countries wouldn't support them.
SO -- for * political reasons, for * economic reasons, and probably out of * racial prejudice as well, many white US conservatives hated Mandela and all he stood for. They weren't necessarily stupid to do so, for Mandela threatened an established system of racial & economic injustice that they found useful and profitable.
Of course it WAS stupid for US conservatives to think that a white apartheid government that represented only 20% of the South African population was going to last for very long. No matter what the US political and economic elite believed, Mandela was on the winning side of history.
And since he turned away from Communism and did very little to correct gross economic injustices in South Africa in the 1990s, after he was elected President the smarter American capitalists -- Republicans & Democrats alike -- are falling over themselves to say how great the man was.
Because some of the gross economic and racial injustices that made South African such a great place for foreign investment in the 1980s still exist today.
-- from a US democratic socialist /former anti-apartheid activist