Liar Selfish and Chaotic
I think that Don John is motivated by the fact he is a bastard. Throughout his life he was told that he is bad and eventually he just became bad. He was probably outcasted because he was a bastard and that had negative effects. The quote "I cannot hide what I am"(act 1 scene 3 12-13) shows that he thinks he is evil and he doesn't think he should try and change that.
Don John doesn't really change from the beginning of Much Ado About Nothing to the end of it. Throughout the story other people would really treat him like and outlast and they talked about him like he was evil. They did this because of the fact he was a bastard which caused him to be more evil. "...Don John your brother incensed me to slander the lady hero, how you were brought into the orchard and saw me court margaret in Hero's garments, how you disgraced her when you should marry her"
Don John contributed to the plot by being the antagonist in the story. He tried to stop Claudio and Hero from being married. He did this by asking Borachio to convince Claudio that Hero had cheated on him.
He impacted the theme of love by being the antagonist also. Since Don John still loves chaos seen when he was cheering after his plan worked. Don John really conflicts with what the theme would be without him because at the end he doesn't have a happy ending.