He went on a rampage that saw him murder four people, including Anna Moiphitlhi, 31, and her three-month-old daughter Elizabeth, Enoch Motshelanoka, 10, and Sivuyile Peyi, 36. Another 11 people were injured.
A total of 125 spent cartridges were recovered from the scene, and he even paused at one point to ask a neighbouring farmer for more ammunition, shooting one of his ostriches when he refused.
Tensions were evident at the judicial proceedings as his trial opened at the Mmabatho high court.
Nel arrived under heavy police guard, and there was a substantial uniformed presence even inside the courtroom itself, which was packed with relatives of the dead.
Nel had remained defiant throughout his preliminary court appearances – one of which found him mentally fit to stand trial – and showed no emotion during the latest proceedings. But in a surprise move he pleaded guilty to 17 charges, among them multiple murder counts. One earlier charge of malicious damage to property, relating to the ostrich, was withdrawn.
His lawyers are now expected to make pleas of mitigation, and he will be sentenced later.
The killings have raised the spectre of ongoing racism in the country, particularly as Nel would have been barely of school age when apartheid ended 14 years ago.
On his blog, Ray Hartley, editor of South Africa’s Times newspaper, wrote: "Nel was born after the release of Nelson Mandela and the unbanning of the ANC.
"He was schooled while Mandela was president under a curriculum reformed to properly reflect South African history.
"How this youngster came to harbour such deep-seated racism is a concern for this society which so badly needs to move on from the past.
"The Skierlik killings suggest that the seed of racism has been planted in the minds of some of the next generation of our youth."