In “To the Reader” from Flowers of Evil, Baudelaire says that Ennui, or boredom, is the worst sin.

Boredom is the worst sin because it is the father of other sins.  In other words, when people are bored, they commit sins.  Baudelaire notes that while murder, rape, and arson are terrible, Ennui is worse.

He is Ennui! — His eye watery as though with tears, 
He dreams of scaffolds as he smokes his hookah pipe.
You know him reader, that refined monster,
— Hypocritish reader, — my fellow, — my brother!

He would “willingly make of the earth a shambles.”   When Baudelaire says that a yawn can “swallow the world” he is reminding us of the dangers of boredom and the trouble it can cause.  He also notes that everyone is guilty of boredom, and if we claim not to be than we are hypocritical.