This is from "Manfred," a three-act dramatic poem (like a play) written in 1816–1817 when Byron was 28 years old. The quote comes at the end of the first scene of act III. Here are the last 40 lines of that scene:

ABBOT: This should have been a noble creature: he  
Hath all the energy which would have made
A goodly frame of glorious elements,
Had they been wisely mingled; as it is,
It is an awful chaos-- light and darkness,
And mind and dust-- and passions and pure thoughts,
Mix'd, and contending without end or order,
All dormant or destructive. He will perish,
And yet he must not; I will try once more,
For such are worth redemption; and my duty
Is to dare all things for a righteous end.           
I'll follow him-- but cautiously, though surely.