The American state of Alabama has put to death a prisoner who over many years showed signs of mental illness – in spite of the US supreme court outlawing the execution of mentally ill people on the grounds it is unconstitutional.

Alabama has executed Christopher Johnson by lethal injection for killing his infant son. (Photograph: Alabama Department of Corrections) Christopher Johnson died by lethal injection at Holman prison in Atmore, Alabama.

The Equal Justice Initiative pointed out that Johnson was in and out of psychiatric hospitals throughout his childhood and was put on anti-psychotic drugs. On death row he kept away from other inmates, spent all his time alone in his cell and showed erratic behaviour including attempting to kill himself by eating toilet paper and slamming his head against the cell wall.

Several US supreme court judgments have underlined that putting mentally ill or incompetent people to death is against the constitution. Yet no court ever evaluated whether Johnson was mentally competent to face execution.

Johnson insisted on representing himself during key parts of his trial. He was put on death row for killing his six-month-old son Elias in February 2005.

After the sentence he consistently declined any offers of legal advice and requested no further appeals against his execution. Academic studies have found that about 80% of so-called "volunteer" cases like his – where the prisoner voluntarily brings on his own death – show signs of serious mental illness.

Opponents of Johnson's death said that it amounted to state-assisted suicide, which is illegal in the US.

Esther Brown of Project Hope to Abolish the Death Penalty, a group whose board consists of Alabama death row prisoners, said: "Our thoughts and prayers are with Christopher Johnson's family members, who are also the murder victim's family. We will not condone state-assisted homicide, even when the individual requested it."

Johnson is the sixth person executed this year in Alabama, the state with the highest execution rate per capita in America.