State officials had to rely on less traditional sources of medications because it’s hard to find companies that want to sell you drugs that are known to kill (unless they need to get a last pack of smokes for the condemned). Even drug manufacturers have a conscience. So do many medical professionals. Most of the doctors who had previously participated in executions have been shamed away from the practice.
Even assuming the drugs in the mix were prepared properly, and the IV inserted the right way (apparently not a given), the cocktail sounds unreliable. The sedative midozolam doesn’t have a standard dose; when used for sedation it is usually dosed “for effect”. If the patient is still freaking out after a couple of milligrams, you give them a little more.
I can think of only one reason to use both potassium (which stops the heart) and vencuronium (which paralyses the muscles including the ones responsible for breathing). With the “patient” paralyzed properly the observers don’t have to watch the person writhe in agony as the notoriously painful potassium runs into the veins.
There is no way to execute a prisoner with medicine in a way that is not cruel and unusual. Doctors, who could increase the “success” are ethically forbidden from participating. The drugs are painful and unreliable. The practice must stop.