"O gentle child, beautiful as thou wert, Why didst thou leave the trodden paths of men Too soon, and with weak hands though mighty heart Dare the unpastur'd dragon in his den? Defenceless as thou wert, oh, where was then Wisdom the mirror'd shield, or scorn the spear? Or hadst thou waited the full cycle, when Thy spirit should have fill'd its crescent sphere, The monsters of life's waste had fled from thee like deer.
The "trodden paths of men" are the places that we humans go. Urania wonders why he had to leave these earthly places so soon.
Here, she even blames Adonais-Keats a little for his death, asking him why he tempted the "unpastur'd dragon" (unpastured means "set free") in his den. Here, Shelley is using a metaphor; the dragons are the critics.
He says that Keats was defenseless against these critics, and didn't have wisdom to act as a shield, or scorn to return their criticism.
Shelley thinks that Keats should have waited "the full cycle" before releasing his work to the public. Basically, he feels that Keats should have matured a little so that he could have handled criticism better. If he had waited, he would have made these critics flee "like deer" from him.