Celebrities who commit crimes are more fascinating than normal celebrities because we just don't understand why someone who has it all would throw it all away. There's also a lot of people who like to see people fail and their general obsession over celebrities would even lead them to scrutinize a celebrity who jay walked.
Yes, we are too obsessed with celebrities who commit violent crimes, really too obsessed with all celebrities. It's one thing to have heroes and people to look up to for their actions and abilities, but to obsess with a celebrity for their wrongs or for celebrity in general is not only rediculous but downright dangerous.
It's interesting to see stories about celebrities, not just commiting violent crimes. We idolize celebrities and often feel that their lives are much better than ours. When they commit crimes like this, we see that they aren't so much different than we are, and I think that is fascinating to most people. It isn't really an obsession, just interesting to see the similarities.
Celebrities that commit violent crimes are fascinating because they make us question why they would do such a thing. People don't understand why someone with fame and fortune would need to commit a violence crime and hurt others when they have so much to begin with. It makes us question our values.