What does "moral vision is so imperiously unsparing" mean?

Solution 1:

The statement in bold simply means that the poet has a strict sense of what is "right".

The words "imperiously" and "unsparing" means a strong person in terms of vision and decision making when it is used for leaders, famous people etc.

Solution 2:

Imperious : marked by arrogant assurance

Unsparing : not merciful or forbearing

The author is saying that Hill is so sure of the validity and superiority of his own moral judgements that he casts his gaze about to comment on whatever he sees that does not meet his standards. And that there are not very many people who do this with as much arrogance and lack of regard for people's feelings or willingness to indulge other's views.