It is not good, i.e. it is a most wicked and hateful thing. See Poole on "Proverbs 17:15,26. To accept the person; to favour his unrighteous cause.
Related Commentaries of Proverbs 18:5
Proverbs 18:5
5 It is not good to accept the person of the wicked, to overthrow the righteous in judgment.