If someone asked me why legalism is destructive, I might give the following reasons.
1. Because it undermines the cross of Christ. If you can save yourself through rules, what’s the point of that blood-stained tree?
2. Because it produces arrogance on behalf of those who seek rule-keeping as the basis for salvation.
3. Because it produces shame and guilt for the recipients of legalistic oversight.
4. Because it disregards the sufficiency of Scripture, thus making oneself, not Scripture, one’s ultimate authority and guide in life.
5. Because it doesn’t produce the inner joy, peace, and satisfaction that true Christianity affords.
6. Because it makes Christianity seem like a scientific textbook to follow, as opposed to a personal, vibrant, joy-filled covenantal relationship with the Triune God.
7. Because it motivates obedience through impure and wrong motives.
8. Because it promotes self-justification and ultimately self-worship.
9. Because it isolates various biblical pages, and translates them without regard to literary and historical context.
10. Because it doesn’t place the beauty, majesty, and glory of God as one’s focus, but instead oneself.
11. Because it promotes strict obedience to rules (according to the legalist’s personal interpretation) but not the spirit with which the rule is given.
12. Because it produces control, removing yourself from having to wrestle with the sovereignty and wisdom of God.
13. Because it doesn’t seek holiness by the power of the Holy Spirit, but through one’s effort alone.
14. Because it doesn’t promote the fruit of the spirit, but instead lifeless religion.
15. Because it can lead to hell.
Those are just a few that come to mind.