Is it considered disbelief for a person to hate a sin in their heart, yet say "there's no harm in it" with their tongue when committing it, and is that considered as legitimizing the sin?

Disliking sins is a sign of faith, and the phrase "there's nothing wrong with it" is erroneous. However, this does not necessarily mean that the speaker deems the unlawful permissible. Deeming the unlawful permissible occurs by believing that Allah has not forbidden it. A specific individual is not to be declared a disbeliever except with definitive proof and after the establishment of evidence against them, because whoever enters Islam with certainty does not exit it except with certainty.