You would argue that the good are our friends and the bad our enemies?

Yes.

And instead of saying simply as we did at first, that it is just to do good to our friends and harm to our enemies, we should further say: It is just to do good to our friends when they are good and harm to our enemies when they are evil?

Yes, that appears to me to be the truth.

But ought the just to injure any one at all?

Undoubtedly he ought to injure those who are both wicked and his enemies.

When horses are injured, are they improved or deteriorated?

The latter.

Deteriorated, that is to say, in the good qualities of horses, not of dogs?