Bad or wicked things can be described as sinful. You might object to a greedy businessman's sinful mistreatment of his employees , for example.
Immoral or ungodly acts are considered to be sinful by some religious believers—so when a sin is committed (like telling a lie or killing someone), the behavior is described as sinful. You can also use this adjective to talk about evil acts, even if you're not sure they officially qualify as sins, or even to mean "excessive." An extravagant dessert, for example, might be called "sinful triple chocolate cake."