I wouldn't worry too much about it... A lot of people have those rape fantasies, and many people are also looking at things like that not somuch because they're into it, but moreso that it represents something taboo, which in itself interests them...

I'd bring it up jokingly, like, "What's up with all this hardcore shit on your computer? Never thought you'd be into this kinda stuff?"

You should be able to gauge him based on his reaction, whether he's fairly serious about being "into" it, or whether he just says it's some of the weirdest hardcore stuff he's found on the net...