Even though Ravana was among topmost in Bhakti towards Shiva the entire clan is branded evil for misdeeds that he alone was responsible. 

Vibhishana was a god man of morals, could rule well not necessarily due to alliance with Rama. MahaVishnu sent the Rakshas king MahaBali to the nether world (Paathaala) even though his grandfather Prahlada was his greatest devotee, and for what misdemeanors no one knows ..at least not properly projected in the popular Puranas.

The Asura by default is a bad character. It is always that the Lord has to take up an Avatar from time to time to slay an otherwise invincible Rakshasa. Devas are generally depicted in Hindu mythology as average pious lot but needing to seek out help from the Trinity when the Asura originated trouble goes beyond a certain threshold.

Human nature is naturally ingredient-ed with piety and evil. Is it from the nature and behavior the good/bad boy branding is done? When the Devas and Rakshasas have a common hereditary lineage  does behavioral leaning really set them apart?