According to Baudelaire, the worst sin is Ennui, or boredom.

In “To the Reader” Baudelaire does not paint a happy portrait of human nature.  We are all headed straight to Hell, and get closer and closer every day.  Yet Baudelaire chooses an interesting sin for the worst one.  He comments on rape, arson, murder, and other sins, and then says there is another more terrible than the rest.

There is one more ugly, more wicked, more filthy! 
Although he makes neither great gestures nor great cries, 
He would willingly make of the earth a shambles 
And, in a yawn, swallow the world

This sin is Ennui, which he capitalizes.  Ennui means extreme boredom.  Why is this such a terrible sin?  Well, when you are bored it leads you to commit other sins.  In other words, idle hands are the Devil’s playthings.  When we commit the sin of boredom, it leads us to commit other sins.

Described as a "refined monster," Ennui is a uniquely human trait.  Animals do not usually get bored.  Humans get bored, and this is why humans are such trouble makers.  Our boredom brings other sins.