You need to honor your parents not because you think they deserve it, but because God asked you to. By doing so, you honor God by keeping His commandments.
You may feel that by honoring your parent, you are excusing his behavior. But remember that no evil will escape God’s eyes or go unpunished. Romans 12:19 says, “Never take your own revenge, beloved, but leave room for the wrath of God, for it is written, ‘Vengeance is mine, I will repay,’ says the Lord.”