He was right. To stop him, security police were forced to kidnap the seriously-ill Congress of SA Students member, drug him and then execute him with a shot to the head before burning his body.
At the time of the photograph, Mtimkulu had just returned home to Port Elizabeth from two-and-a-half months' treatment at Groote Schuur Hospital in Cape Town for thallium poisoning, allegedly administered while he was in detention.
Mtimkulu disappeared two weeks after initiating a R150,000 civil suit against the minister of law and order, alleging he had been poisoned in detention.
Next week four former security policemen, including retired general Nic van Rensburg, go before the Truth and Reconciliation Commission's amnesty committee in Port Elizabeth for the murder of Mtimkulu and his friend Topsy Madaka.
Last April, Van Rensburg and Brig Jan du Preez filed an urgent High Court application to stop the TRC from hearing evidence which implicated them in Mtimkulu's death.
Judge Edwin King ruled that the two should be given "proper, reasonable and timeous" notice of the TRC's intention to hear evidence of human rights abuses in which they were alleged to be involved.
Mtimkulu's mother, Joyce, finally got her chance to testify at a special hearing later in the year.
Now her son's killers are preparing to tell of his final hours before he was shot, his body burnt and the remains thrown into the Fish River.
However, she does not believe they will tell the whole truth, since they deny poisoning him.
Jan Wagenaar, the lawyer for one of the applicants, told Sapa the policemen would be "stupid" to lie about the poisoning if they had knowledge of it.
"If they are asking amnesty for murder and they were involved in the poisoning they would be stupid to leave that and ask for amnesty for the more serious crime of murder."
The poisoning of the student activist made headlines after his release from detention at Jeffreys Bay police station in October 1981. Mtimkulu complained of pain in his legs the day after his release.
He was admitted to Port Elizabeth's Livingstone Hospital on October 26 and later transferred to Groote Schuur Hospital in Cape Town where thallium poisoning was diagnosed.
In its inquiry into state involvement in politically motivated violence in 1990, the Harms Commission was told that Mtimkulu appeared "healthy and happy" after his release.
This observation was recorded by police colonel Jacobus Barend du Plessis, who denied giving the activist three kinds of pills during the last month of his five-month incarceration.
In his testimony to the commission, former police forensics chief Gen Lothar Neethling denied supplying thallium to a Brig Van Der Hoven, who then allegedly took it to the Eastern Cape.
The general said the poison, used in any analytical laboratory, was a central nervous system suppressant, which, if administered in a large enough dose (one to two grams), could lead to death within a week, he said.
It had been used as rat poison and as a method of treating hair loss and skin problems in the 1950s. It had also been a popular means of criminal poisoning.
Thallium, a colourless, odourless and tasteless chemical, is also used in the manufacture of lenses, imitation jewellery and alloys.
In statements after his release from detention, Mtimkulu described his deteriorating condition:
"The pain got so bad that I was reluctant to walk. I had to crawl to my parents' room for attention. I felt a continuous pain in my feet and lower legs. I could not even find a comfortable position to sleep in bed."
Prof Francis Ames, former head of the department of neurology at the University of Cape Town, treated Mtimkulu and recorded his condition in a case study published in the SA Medical Journal of August 1983.
She noted that he was unable to eat solid food, vomited frequently and could not sleep at night.
He had told her of abdominal pain that started several months before his discharge from prison and was accompanied by constipation and rectal bleeding.
A urine analysis indicated the presence of thallium. A British expert who confirmed the finding said the results suggested the "ingestion of an appreciable quantity of thallium".
Ames' study also noted: " He was booked for readmission (to Groote Schuur) three-four months later, but failed to arrive. We were told that he had suddenly disappeared shortly before his admission date.
"There has been no trace of him since that time, so we do not know whether the neurological damage is permanent."
By that time, Mtimkulu was already dead, shot with a single bullet to the head.
On April 14 1982, Madaka picked up Mtimkulu from home to take him to Livingstone Hospital, as Mtimkulu was by then unable to walk without assistance.
It is known now that the two men were abducted by security police outside the Port Elizabeth Holiday Inn. Their families never saw them again.
In a 1990 interview Mtimkulu's mother Joyce said: "If we can just get a bone, or a nail, which we can say is Siphiwos's. Until then we can't forget.
"If Siphiwo is alive in this world then I know he is not happy and is still suffering."