This. I've always wondered why they don't use heroin, or morphine. IV ten grams and it's bliss, nod off and then complete respiratory shut down. Quick, easy, painless, and free.
That IS effectively what they used. In this case, the lethal injection was a massive OD of hydromorphone (a synthetic drug like morphine, but quicker acting and more potent).
Just to be sure they also gave a big dose of a sedative (midazolam, a ultra fast acting benzodiazepine - like valium, or attivan, but more potent) at the same time.
The outcry in this case appears to be that the death lingered for longer than expected, and that they could see the dying gasps of the victim. This is not unexpected, death can take a considerable period of time following even a massive overdose.
Traditional execution lethal injections get around this by giving a paralysing agent at the same time. This completely paralyses all muscles, so that the victim stops breathing, and the common gasping-type breathing (air hunger), muscle twitching and muscles spasms that you get at death don't occur. When someone suffocates, which is basically what happens in lethal injection execution (the agent, an anesthetic or similar drug, essentially slows breathing to a non life sustaining rate).
The traditional cocktail then finishes the job with potassium chloride which stops the heart within a few seconds. Again, it was odd that they omitted this, as it could easily be prepared by an pharmacy.
While claims that this method of execution would have resulted in excess suffering by the victim, are patent nonsense, the fact that the execution took a long time, and looked violent and distressing, is not a surprise, and suggests that the people designing the method really didn't understand what they were doing.