I believe it is decidedly sinful, to watch shows which present evil things that go directly against what God has taught in His Word as entertainment. I am saddened that most modern churches, whatever the denomination, refuse to address sin and call it sin. Instead, we tread on eggshells in church, afraid to offend anyone with a strong stand for truth. We are called to be holy and blameless, how then can we do 75% of the things that we, as Christians, carry on doing?
I agree, if you are watching shows that condone, better yet approve, evil and tries to foster it. However, I watch many shows/movies that have evil in it, yet the writers and directors also show how miserable these people are. So, for those shows it is hard to say it is decidedly sinful. We can learn things from TV shows and movies, even if they are secular.
I like Dexter. A Showtime show about a serial killer. Yet, he himself only kills serial killers. A great show, IMO. Good writing. Season 1 was the best. Another show I like is Six Feet Under, a HBO show, and as the name suggests deals about death. There is a lot of nihilism b/c some of the actors are atheistic and some are Christian with different views. Still one can learn about the struggles and questions we all deal with in life. The pain, loneliness, desires, and consequences (both good and bad) of our actions.
Yet, many people would consider those shows to be 'evil.' I don't.
You say we are to be holy and blameless. I agree, but don't let it devour your life.
"Do not be overrighteous,
neither be overwise—
why destroy yourself?" (Ecc 7:16)