It is illegal to euthanize humans, so the choice of 'putting them to
sleep' vs morphine never comes up. But, having worked as a nurses aid I
have seen humans essentially overdosed on morphine to death, and I've
also seen humans 'maintained' on morphine while they more slowly died of
dehydration or starvation. Or, kept on morphine while they die of
whatever is ailing them. (These people did not seem to have a bad or
painful death.

Such humans dont get their morphine orally nor even by shots, but get
the drug administered via IV.

But, if you're gonna give an animal a 'shot', it doesnt make sense to
give morphine when you can simply give them the normal (?) shot that
puts them to sleep.

I had a cat whom I took to the vet last year thinking it might be for
euthanasia; he was very old and had been very sick for a very long time,
and seemed to be nearing the end. However, on the way to the vet,
somehow he KNEW, and he began looking me at imploringly, pleadingly, and
I couldnt do it. He ended up living another 4 weeks, mostly spent
peacefully sleeping; he didnt seem to be suffering or in pain, and in
the end, he just went to sleep one night and didnt wake up. I was very
glad I left it up to him as to when to go......

But, now I have another very sick cat, this one, alas only about 3 years
old, and in severe renal failure.
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I would prefer for her to guide me, but, if I have to make the decision,
if she is suffering or in pain, if she has to be put down, I'm thinking
I want it to be done with a captive bolt pistol. I believe, and I have
also heard a vet say, that this is the quickest, least traumatic way of
doing it. It is instantaneous and the animal doesnt see it coming.
I've heard that it can be difficult to find a "pet" vet who has a
captive bolt pistol tho. Has anybody ever had a pet put down this way?