Final answer:
The man's life was principally governed by the pursuit and systematic exercise of revenge, which lead to a dwindling of his other qualities, such as strength and intellect.
Step-by-step explanation:
The principle of the man's life, as stated in the passage, is the pursuit and systematic exercise of revenge. Despite references to his strength and intellectual force, these aspects are not identified as the core principle but rather qualities that were abandoned as a consequence of his singular focus on revenge. This man's devotion to the path of revenge consumed him to the point where all other aspects of life seemed to vanish.