Don't be fooled, as Redcrosse is in Book 1, by this seemingly harmless, unarmed man. Despair is a smooth operator and brings Redcrosse the closest he comes to actually dying before Una saves him. What's particularly scary about Despair is that he doesn't actually do anything, he just slyly convinced people who talk with him that life isn't worth living and that they should kill themselves.
Redcrosse is skeptical that "idle speech" (I.ix.31) could have any real power, but boy, does he learn that it does. After chatting with Despair for only a little while he gives up all hope of living in this world and prepares to stab himself with a knife.
Luckily Una, and her promise of truth, prevent Redcrosse from this terrible act, but it takes him a long time in the House of Holinesse to get back to his old self. Since despair represents a complete loss of hope, and since hope is something God gives us, despair was understood to be a complete turn away from God. In the world of the Faerie Queene, that is not a good thing, especially for Redcrosse, the knight of Holiness.