In the poem the author is comparing her experience of killing an innocent non poisonous spider to the police killing innocent unarmed people (suggesting of color because of the "brown" recluse reference). She is trying to explain that killing the harmless spider just because she was scared doesn't make it right and the same thing goes for police and people of higher authority.
A character who drives most of the action in a story is the protagonist. The protagonist is the main character of a story and he/she drives most of the action in a story.
