The dog Dexter shows sinister behaviour (decapitating saints).
I thought you were referring to that TV show 'Dexter'. Its about a serial killer or something.
Decadent means "decaying". The Communists used to call Western society "decadent", meaning decaying, but (some) Westerners have taken to using "decadent" as if it was a good thing - i.e. luxurious.
Decadent means sinful, like, these cookies are so good its a sin to eat them.