"He became a thorn to those who are opposed to workers' progress," Amcu president Joseph Mathunjwa told reporters in Johannesburg on Monday.
"Numerous threats were directed at him, but he was never shaken."
Mawethu Steven (46) was shot dead on Saturday while watching soccer on television at a tavern in Photsaneng, he said.
"We are shocked and saddened by the unfortunate death of our North West regional organiser.
"We will like to call on all our members during this difficult and emotional time to remain calm and allow the law to take its course."
Mathunjwa called for calm, denouncing violence as being destructive.
"Violence deprives us of all the gains which was fought for by our heroes and heroines in achieving the democracy we today enjoy in South Africa," he said.
"Freedom of association is what we fought for in the new democratic South Africa. People should not be killed for exercising their right to associate or disassociate. Let peace prevail."
Expelled
Mathunjwa denied Steven had been vocal in calling for an elective conference, where he was set to challenge him for the presidency of the union.
"Mawethu was an official, not an office bearer, and as such he was not eligible to contest any position, even for being a shop steward."
He said the former National Union of Mineworkers (NUM) branch chairperson at Karee in Marikana was employed as a regional organiser responsible for Lonmin.
Mathunjwa said Steven was expelled by the NUM in 2011 for speaking out against corruption. "He was a leader who had a great passion in the struggle for the betterment of the working class."
At the time of his death, Steven was due to give evidence at the Farlam commission of inquiry in Rustenburg.
It is investigating the incidents leading to the killing of 44 people during last year's wage-related unrest at Lonmin's platinum mine in Marikana.
Police shot dead 34 striking mineworkers on August 16. Ten people, including two police officers, were killed in the preceding week.
Mathunjwa said he did not know the content of Steven's testimony at the commission.
"He was speaking to lawyers. I do not know the basis of his evidence."
He said the motive for the killing was not known and could not speculate that NUM was responsible for the killing. "It is unknown who is responsible for the killing of comrade Steven. All we heard was that four men came to where he was sitting and shot him, and he died instantly."
Armed men draped in blankets
​Amcu could not confirm that the two brothers killed in Nkaneng informal settlement in Wonderkop were their members.
"It is for the first time we heard about the two. We do not know whether they were our members."
North West police said the two brothers were shot by armed men draped in blankets and wearing balaclavas on Saturday.
Brigadier Thulani Ngubane said the group stormed into a shack and asked the twin brothers inside where the other man was.
The brothers were shot when they said they did not know the man. The identity of the man was not known.
"The one brother, who was shot two times, died on the scene, while the other brother died on arrival at Andrew Saffy Hospital."
Two women, who were in the shack at the time, were wounded.
Hacked with a pang
One was shot in the thigh, and the other was hacked with a panga. They were taken to hospital and discharged on Sunday.
Steven was the fourth man to die before testifying at the Marikana commission.
Daluvuyo Bongo, a NUM shop steward, was killed at Wonderkop hostel in October last year. He was to testify for the NUM.
Alton Joja (69), the sangoma who allegedly sent his sons to perform rituals which were meant to make mineworkers "invincible", was shot dead in the Eastern Cape in March. He was to testify for the police.
Rock drill operator Lungani Mabutyana (27) was found hanging from a tree in Wonderkop.
He allegedly committed suicide after he was fired from his job in March and was experiencing financial problems. – Sapa