I am not exactly sure how the eldest brother's death came to be but this is my understanding: he realizes he has been poisoned and assumes he does not have much longer to live. By holding off the advancing enemy soldiers, he can buy Lord Bo the time needed to save the youngest brother. The young one wrongly assumes Lord Bo is a coward who wanted only to save himself. Lord Bo did not tell the young one the truth because he did not want him to feel guilty. As for the poison, it is not made very clear. Later on in the series it is said that the king poisoned everyone with slow acting venom as a means of ultimate control.