Why is Don John full of bitterness in Much Ado About Nothing?

Let us remember what has just happened prior to the
beginning of this hilarious comedy. Don John has just tried to overthrow his brother and
the soldiers that enter in Act I scene 1 have just fought the rebel force of Don John's
soldiers. Don Pedro, Don John's brother, has forgiven Don John and receieved him "into
his graces," yet clearly Don John is overwhelmed by bitterness because of his failed
attempt to seize power. In addition, let us remember that Don John is the bastard child
of his father, whereas Don Pedro is the legitimate heir. Bastard children had no hope of
ever inheriting, and were always doomed to be regarded as being lower than legimitate
children. Thus we have many reasons for Don John to be overwhelmed by bitterness.
However, let us also remember what Don John himself tells us in Act I scene
3:

I had
rather be a canker in a hedge than a rose in his grace. And it better fits my blood to
be disdained of all than to fashion a carriage to rob love from any. In this, though I
cannot be said to be a flattering honest man, it must not be denied but I am a
plain-dealing villain.

So, in
addition to his status as a bastard and his failed revolt, Don John also chooses to be a
character that gives in to negative thoughts and is overwhelmed by
bitterness.