I think the idea that the apartheid regime "collapsed" is an inaccurate description that cheapens what actually happened.

However, it is true that the regime wasn't ended through violence, but negotiation.

The chief architect of the negotiation was F W De Klerk, a white South African, who came from a highly conservative background, it was something of a surprise when he began to call for the end to racist South Africa.

Ultimately de Klerk was a pragmatic realist who understood that time was running out for the Apartheid regime, and from the beginning of his presidency spoke out against apartheid, and took steps towards ending it. One of the first moves he made in early 1990 was ending the bans on the ANC and the Communist Party of South Africa (the communists had been instrumental in assisting the ANC in their terrorist/freedom fighting campaign) at the same time, de Klerk, released many political prisoners, including Mandela. (Those who had been imprisoned for membership of banned organisations, rather than political violence)

The point is, the end of Apartheid was a gradual process, taking about four years, between de Klerk's election in 1990 and the first universally democratic vote in 1994 which elected Mandela's national unity government. Incidentally, de Klerk took the position of deputy president under Mandela, a position in which he remained until 1996. In 1993, de Klerk and Mandela were jointly awarded the Nobel peace prize for their work in ending apartheid and forming the new democratic constitution of South Africa.

That said, de Klerk is definitely not a saint, his earlier politics were mostly pro segregation, if not pro apartheid. Just as Mandela was also not a total saint. Both men were pragmatists, who worked together towards a relatively bloodless ending to a brutal system of oppression, the future of which was rapidly disappearing.