Final answer:
The climax of 'Of Mice and Men' showcases themes of friendship, responsibility, and mercy as George decides to kill Lennie to protect him from a mob, displaying his sense of duty and mercy within the context of their deep bond.
Step-by-step explanation:
The themes of friendship, responsibility, and mercy are pivotal in the climax of Of Mice and Men when George decides to kill Lennie. George's act is a complex mixture of these themes. His friendship with Lennie is evident in his desire to protect him from a more brutal death at the hands of the mob.
The theme of responsibility emerges as George believes it is his duty to spare Lennie from such pain, and in essence, he takes responsibility for ending Lennie's life mercifully rather than subjecting him to the terror and violence of the mob. This act can also be seen as one of mercy, preventing Lennie from enduring the cruelty of others.
The theme of betrayal could be discussed in contrast as some might argue that George betrayed Lennie's trust. Yet, this perspective is nuanced by the extreme circumstances they face and George's motivations to protect his friend.