too dog/under dog

In the documentary, The Topdog Diaries (see YouTube), Don Cheadle explains that an audience member once asked him: What is the real reason that Booth shoots Lincoln. Cheadle does not give a clear answer, instead, leaving it up to the audience member to figure it out.
In your opinion, what is the symbolic reason that Booth shoots Lincoln at the end of this play? Please, think bigger picture than the simple violent scene that the brothers, and we as an audience, observe at the conclusion of this play. Again…BIGGER ISSUES ARE AT PLAY IN TOPDOG UNDERDOG. Think critically to score maximum points!