@Mr Fight the Power,
There is the need to act differently when around other people, when there is social interaction. And what's more, cognitive behaviour is surely different when around other people because they have meaning. Your behaviour is going to be different when around thousands of people vs. a few vs. one other vs. being alone.
If the decision making process is truly deterministic (and I'm assuming there is just not enough time for critical thought of the situation with the grenade or whatever) , then decisions are not based on intention for this scenario. They are actually based on self preservation of the ego.
There can be no time for altering the ego by logic to change the overall outcome/action, because the intent is to satisfy the self preservation of the ego. Cognitive processes all have their role, and the processes have to be limited in this situation, let alone the behaviour to something instinctual. I mean really, how long does it take for a grenade to blow up.
So no, whoever said that decisions are base on self interest are a little off I think. Decisions are based on self preservation of the ego. Self interest is what comes along from external environment, affecting the mind and invoking the desire to change the ego from which your actions will try to be reflected upon.
Now I shall try to incorporate the altruism. When comparing a society when there is an uneven power distribution, the amount of power somebody has can be correlated as inverse proportions to altruism and proportional to ego. And I'm assuming here is doesn't change. Somebody pointed out that the ultimate altruism is that of no self interest. True, but this is only because there can be no egoistic frame to evoke the self interest. So I cannot say that just because there is not logical input that there is no altruism available. I cannot refute the idea that altruism is an irrational, emotional, or instinctual possibility. I'd lean more to the instinctual, but I have no way of objectively proving this. If emotional then this topic is useless because it will just highlight and conclude the same point Didymos made from the very beginning, in other words(because I don't remember what was said exactly) , we cannot concede a possibility of someone else's subjective experiences.
So altruism isn't this magical event. It may appear that way, through the sordid, involuntary idea that altruism being inversely proportional to ego in human behaviour via the imbalance of power scenario should cause such misconceptions, but we can never get rid of the morality of the situation as a description. Morality should have become an inherent way to satisfy the ego and self interest. If one were to ignore this, then one would have to consider all the cognitive processes in this scenario as purely rational, probably logical.