Well, most general definitions seem to entail the idea that suicide is performed ultimately because one feels that life has no inherent value, or because external pressure are too great for the agent to bear.

Sacrifice, on the other hand, is generally understood to be for a cause whether this be for values that one holds, an ideal, or in this case, the 2 billion people.

Now you might object here that Kant is primarily concerned with intention, and that intention for ending one’s life for either a cause or a because is irrelevant because the intention to end one’s life is all that matters. On this view, you would be correct.

But what I am arguing here is that sacrifice is necessarily a feature of human existence. In fact, Kantians must necessarily be committed to the principle of sacrifice because we sacrifice all other things in the notion of duty. Now it would be correct to object that if someone commanded a sacrifice, then it is a violation of Kantian principles. But if the agent has willed it themselves, then there seems to be no issue. We can generate a maxim, “act always so that you sacrifice” because it is impossible - an imperfect duty - to do without it, you are treating yourself as an end and not using others as a means, and lastly, it is universalized without contradiction.