I believe the definition itself is a temporary social construct. Let's remember that not too long ago, homosexuality was classified as a mental illness. What I think remains the same is that there are certain people who enjoy what others consider perverse, and perhaps simply because they are a minority, they are labeled as sick. Let's take BDSM practitioners as an example. Is a masochist sick, or do they simply have very niche tastes? But when it comes to killing for pleasure, I don't think that's a pleasure; there must be some kind of flaw or illness involved, in my opinion.