Answer: The quotation from the passage that best supports the theme that feelings of guilt are difficult to escape is "O coward that I am, to live so long, / To see my best friend ta’en before my face!".
Step-by-step explanation: The passage provided belongs to William Shakespeare's Julius Caesar, which focuses on the conspiracy to murder Julius Caesar. In the play, Cassius, who is one of Julius' longtime acquaintances, is also part of the conspiracy. After the Roman general is murdered, Cassius cannot stop feeling guilty about it. He expresses his guilt when he calls himself a coward in front of Pindarus ("O coward that I am") for allowing his best friend to be murdered ("To see my best friend ta’en before my face!").