The play questions the legitimacy and fitness of King John being the king of England. Arthur was the son of the previous king's eldest brother, making him the rightful heir, but John was chosen to rule by the previous sovereign. Yet in the case of the Bastard, John rules that a will cannot take precedence over the law; in that case, the father's will that his younger son receive the inheritance was overturned by the law, which stated inheritance must go to the eldest son, bastard or not. By ruling such, John unwittingly proves his own illegitimate hold on the throne, because it is based on will and not the legal right of succession.
Shakespeare proves that John is not the legitimate ruler, yet the question is complicated in the clear difference that develops between the idea of "legitimate" and "fit." Arthur is the legitimate ruler, but his portrayal as a weak child under his mother's thumb shows him to be unfit; that is, he would be a weak and ineffectual king. Because John is a stronger man, his claim on the throne begins to seem much more attractive.
This situation all gives rise to a kind of defense of illegitimacy. Toward that end, the Bastard develops as the most compelling character in the play. He enters less as a character than as a set of theatrical functions, embodying the mischievous vice figure of earlier English morality plays. He speaks to the audience and makes observations about events. Yet by the second half of the play, he becomes unswervingly loyal to the king, denouncing deals made between John and Philip, and between John and Pandolf, and criticizing the royal desire for "commodity" and self-interest. The Bastard seems to believe that Arthur's death was an accident and returns to John to defend the crown and kingdom. At this point he becomes both the rhetorical and ethical center of the play.
By supporting John, the heroic and honorable Bastard makes it look like John must be the right choice for king. But ordering the death of...