Some torrents are illegal due to copyright laws, if they are shared without the copyright holders permission. That would be a sin, as it is stealing.
Here's the opinion of a musician friend of mine posted on Facebook a couple weeks ago, regarding Spotify and online streaming. He doesn't talk about torrents, but something else to think about: (I didn't include the photo he took of his statement, but he tells you what it is.)
"So, i'm only posting this because people ask me all the time about Spotify and the royalties artists see. Here is my latest statement from the 4th quarter of 2012. As you'll see one of my songs got 13,750 plays during this time which yielded $7.20. I can't help but wonder if only 1/3 of these plays was a purchase on ITunes and the artist yielded half of what ITunes charges. I only post to answer the multiple questions i receive about it, not to make you feel guilty. Heck, artists have to sign up for Spotify themselves for it to be streamed, which does require the artist to know what they are getting into. It's a promotional tool, plain and simple. However, I would humbly suggest using Spotify and online streaming wisely. It can be a really helpful tool to explore, promote, etc. Like most things (or all things), it can be abused. If one uses online streaming as their primary library or source for enjoying art...it's likely the artists you enjoy are feeling the heat because of that. Just food for thought from a certain perspective."