Um, I can rationally justify self-sacrifice, as far as morals are rational.
1. You're a human being, the difference between you and an animal is that you're not completely driven by your instincts but much rather have a choice. You're not willing to give this difference up, even if this means your death.
2. Who are you to put your right to live over the others rights to live? You can't, therefore you choose to kill yourself in order to save the rest.
3. Who are you to judge who's going to live and who's not? Depending on your world view, you might see this as God's duty or arrive at 2.
As you can see, there is not just one answer, that can be right. It depends on how you think.