When students of the US death penalty look back on the year 2014 they are likely to remember it as one of the most grotesque on record, punctuated by a series of botched executions in which prisoners writhed, gasped and groaned for lengthy periods on the gurney.

But the Death Penalty Information Center, a leading chronicler of capital punishment trends in the US, notes in its annual review published on Thursday that the year was also marked by the onward decline of the controversial practice. The 35 executions carried out in 2014 marked a 10% decline compared with the previous year, and a dramatic slump from the peak of 98 judicial killings in 1999.

Though many people around the world think of the death penalty as being an American foible, the annual report points out that it has receded into a rump of hardline states. All 35 executions were carried out by just seven states, and of those 80% were accounted for by just three states – Texas, Missouri and Florida.

Even in Texas, long ground zero of the death penalty in America, the number of executions sharply declined this year, from 16 in 2013 to 10 in 2014. Only Missouri bucked the trend – it lethally injected 10 prisoners compared with just two the previous year, as it forged ahead with an aggressive new drive to carry out executions at a rate of almost one a month.

The number of new death sentences also showed a steep decline, falling to 72, the lowest number since the death penalty recommenced in the modern era in 1974. The center notes this was the fourth year in a row with fewer than 100 sentences, compared with figures above 100 for every year between 1974 and 2010.

“The relevancy of the death penalty in our criminal justice system is seriously in question when 43 out of our 50 states do not apply the ultimate sanction,” said Richard Dieter, the center’s executive director and author of the report. “The US will likely continue with some executions in the years ahead, but the rationale for such sporadic use is far from clear.”

Opponents of the death penalty will be heartened that its overall trajectory is steadily moving in their direction. But if 2014 is anything to go by, as capital punishment becomes less common, it also appears to be growing more extreme and arguably inhumane.

DPIC’s report highlights the string of botched executions that occurred through the year, starting in January with that of Dennis McGuire who gasped and snorted for 15 minutes in front of his horrified children. It was Ohio’s first – and critics said experimental – use of a new lethal drug combination.

Then in April, Oklahoma saw gruesome scenes of officials struggling and failing to find a vein in death row inmate Clayton Lockett, who struggled for 43 minutes, much of it behind a curtain that prevented the media witnessing the events. New documents released this week unveiled new details of the tragedy, including blood splattering over the doctor who was trying to insert the IV.

The Guardian has joined the ACLU and Oklahoman news outlets in legally challenging the state’s refusal to allow full public access to the execution.

Arizona added its name to the grisly list of death penalty states carrying out botched executions in July when it took almost two hours and 15 doses of drug to kill Joseph Wood.

Part of the reason for the apparent increase in executions that have gone wrong is the scramble for lethal injection drugs. The European boycott of medical drugs going to US prisons is now biting deeply, and in an attempt to circumvent it many states have attempted to find new and ever more unconventional supply routes.

Many states have also tried to avoid public scrutiny of their experimental new methods by passing secrecy laws designed to keep the identity of its suppliers hidden. The Guardian is involved in legal challenges to such secrecy in Arizona, Oklahoma, Missouri and Pennsylvania.

The other area highlighted in DPIC’s annual review are exonerations, with seven former death row inmates being freed in 2014 – the highest number since 2009.

The seven included brothers Henry McCollum and Leon Brown, who were convicted of murder and sentenced to death in 1984 when they were both teenagers. They were eventually exonerated 30 years later when the North Carolina Innocence Commission recovered crime-scene material which provided a positive match to a known sex offender living just a few feet from where the victim’s body was found.