1. Because showing proper respect to everyone is a commandment of no less than God Himself (1 Peter 2:17)
2. Because we are to love the brotherhood of the saints (1 Peter 2:17).
3. Because our failure to do this is an act of unfaithfulness to the Lord (Nu 5:6).
4. Because the small speck of Ken Ham's doctrinal and scientific aberrancy is not worth comparing to the redwood forest of heresy that protrudes from your own soul (works righteousness, belief in a spiritual rather than physical resurrection, denial of Paul's apostolicity, approval of grievous sexual sins, making Christ out to be a new Moses rather than the revealer of grace, soteriological pluralism, denial of the Second coming, denial of the destruction of the cosmos by fire, denial of justification by faith alone, endless idolatries towards theologians...take your pick).
5. Because showing respect for Ken Ham is just exactly what we would wish him to do for us.
6. Because even if he is the most reprehensible sinner alive, he has not yet matched Paul in being the chief of sinners.
7. Because he actually wishes to take the scriptures seriously.
8. Because love keeps no record of wrongs.
9. Because Jesus Christ died on the cross for his sins.
10. Because demonstrating grace to Ken may have more luck in producing the behavior you wish to see in him than fulminating cold blooded and malicious criticisms.