There is a difference between watching sin for entertainment, and watching a movie that depicts sin for entertainment. Most movies are complete works of fiction (or dramatized enough to have little resemblance to the real events). It would be different if we were watching somebody actually be killed, for instance. I would also suggest finding a movie, play, Broadway show, etc. where there is no sin depicted anywhere. I’m sure there are some, but it would eliminate 90% if not more.
For example, almost all Disney movies have sin depicted in them. There is usually a “bad guy” who is grossly envious, desires wealth/power/fame for themselves and is willing to do anything, including attempted murder, to achieve it. Cinderella (Stepmother), Aladdin (Jafar), The Little Mermaid (Ursula), The Lion King (Scar), Beauty and the Beast (Gaston)… just to name a few, all have their sinful villains and in each one, we’re cheering their demise.
This is also different than if the sin IS the entertainment. Watching gladiators be killed at the Colosseum, or watching pornography, or watching somebody’s videotaped crime spree would be sinful if you were watching this for pleasure. Watching Christ be crucified, or any criminal be executed, and doing so with an attitude of revenge or justice would fall under this category. The fact that the sinful activities are being promoted and glorified in the context of the entertainment is where the moral dilemma comes in, but not the fact that the entertainment merely contains sinful activities.
The reason I’m defending the movies in this case is that when this question comes up, people aren’t really asking “Is watching a violent movie a sin?”, but really the question is “Is there a way I can call certain movies sins, and feel self-righteous by avoiding them?” Some groups of people have indeed decided that if watching a sin is a sin, then watching ANY sin is a sin, not just certain ones. These groups often have total bans on movies, TV, music, books, etc… It’s just a slippery slope wherein if watching a fictional sin is a sin, then watching a real life sin also must be a sin. Therefore since sin exists in society, you should just never leave your house for fear of sinning by proximity.