I’ve had two euthanized. One with sedation, one without. The one without was more peaceful, and faster than the one with. But I still feel the one with sedation went quickly, and have no complaints.
I was there for both, so they’d have some familiarity to the end (I kept my hand near a nostril). But on the advice of my first vet I left before the deadstock truck showed up both times. I know the essential spirit that was my horse was gone, but I don’t need to see their shells being handled like the large, heavy, unwieldy objects they had become. I don’t want that memory.
I don’t particularly want the memory of collapse either, but my first horse had a fear of vets and I would not leave him to face that alone. It was important to me to be there to reassure them, but I advise letting someone else handle it if you can’t keep yourself calm enough to prevent upsetting the horse.