Witness Reverend Jeff Hood told reporters he saw a man ‘struggling for their life’ for 22 minutes as Smith became the first US death row inmate executed by nitrogen asphyxia
“Kenny just began to gasp for air repeatedly and the execution took about 25 minutes total.”
IIRC, Switzerland used a hood over the patient, or an airtight pod, that was flooded with nitrogen. It pushed out all other gases so the patient just fell unconscious due to hypoxia without the discomfort caused by CO2 poisoning.
The execution method used a mask, like a rebreather. The nitrogen delivery was inadequate and the convict was able to breathe in some oxygen, either from less-than-pure nitrogen gas, or gaps around the mask.
Active euthanasia is illegal in Switzerland (administration by a third-party), but supplying the means for dying is legal (assisted suicide), as long as the action which directly causes death is performed by the one wishing to die. Source
Nitrogen gas would be administered through a facemask while the person is restrained (essentially suffocation) ... so hardly a peaceful death by choice.
People who are being euthanized are doing it voluntarily and will breathe intentionally to get the peaceful effects of losing consciousness and drifting off. They won't be panicking, or trying not to die.
I assume they also have the option to stop at any time, so they are fully in control of the process.