It is premeditated. A diver making such a decision would probably be thinking about it very carefully. No one would want to let a buddy die. But, people have done just that on several mountain climbs.
Teams of climbers have passed sick, injured or dying climbers in places like Everest because they knew, rationally, that they couldn't help.
Whilst true, the analogy is not quite 100%, Trace.
A lot of the issues, particularly with Everest, is from commercialism and lack of inter-team (as opposed to intra-team) spirit. And the fact that there are multiple teams on the mountain at the same time.
Above about 7000m, you are dying. It is impossible to live at those altitudes. I have no doubt that I would just walk past someone who was not on my team if they were having trouble because I have a finite amount of time before I start to exhibit signs of either pulmonary or cerebral oedema. I can say that with absolute certainty, as I've done it - not my team, not my problem.
At the same time, I've also extended how long I stay at those altitudes (above 7000m is fondly known as the death zone) to assist someone on my team. We ended up camping an extra night because they were exhausted, it wasn't the right decision - my partner developed pulmonary oedema overnight. Trying to sleep whilst listen to someone gurgle as they breathe, wondering whether they will still be alive in the morning, wondering whether you are going to live - it's not a nice feeling. But I never would have left them, not until they were dead.
Some of the larger expeditions are technically "the same team" but they are paying customers who don't know each other - I would never climb on a trip like that, it's insta-buddy and everything that is wrong with it.
I think that's a totally different scenario. Climbers have the time to think, assess the situation, re-assess the situation, do what they can, talk, etc. OOA divers don't have this luxury.
The analogy is not 100% aligned, but you'd be amazed at how little time you do have - particularly at higher altitudes. Plus you are not rational, your brain is starved of oxygen. The number of hallucinations I have had at altitude is... well, it happens a lot and they are very real. In the situation above where I walked past someone who needed help, it was a snap decision - one that I made because my high school Latin teacher was standing next to me at the time, conjugating Latin verbs at the same cadence as my steps. The sheer effort of putting one foot in front of the other is unbelievable. My imagined Latin teach stopped as I looked at the person I was walking past, and said in English "already dead" and carried on conjugating verbs. It was as easy as that.
