i Register
In some senses, sinful is marked as colloquial. Watch for register when choosing this word.
adj
Having sinned; guilty of sin.
Constituting a sin; morally or religiously wrong; wicked; evil.
decadent (luxuriously self-indulgent)
I take a whiff of the most sincerely sinful cookies in the history of the world. It’s a recipe I saw in People magazine once at the dentist's office, and Dr. Segal would not approve of its ingredients (or maybe she would, in the interest of keeping her business afloat).