Sin (maʿṣiyah) is committing a prohibited act while acknowledging that it is prohibited. Declaring lawful (istiḥlāl) is treating what is definitively forbidden as permissible. Doing wrong and rejecting the prohibition are not the same matter.
Sin remains blameworthy even where its commission does not automatically amount to disbelief. Rejecting an established prohibition is a different issue from weakness in practice.
Judgments of faith and disbelief are serious; they cannot be made hastily without examining the actual claim, established belief, and relevant legal conditions.