For starters, its visit is the main theme of the poem. The raven visits and "talks" to the poet. Ravens also have vast symbolism in many cultures, from far northern to eastern ones, usually being connected to things like luck, fortune, sending messages, and similar.
Although many thought knights, to be (excuse me) knights in shining armor. Sir thomas, being a knight himself. Thought of them to be not as everyone thought as honorable. But also as the people they fought. Savages.
It's a matter of opinion but I would say that if there was no evil, good would become the norm and people wouldn't see it as much as when evil is prevalent.
B. his courage
**I may be wrong.**