It's comforting to think that dangerous people often seem weird before they do something evil, but the truth is that many normal seeming people hurt others, and many shy, isolated people would never hurt a fly. Because it is easy to show that the vast majority of people who wear black or watch horror movies never kill anyone, there's no rational basis to find it suspicious. I have seen people say "but a missing child makes it different", but logically, it does not. Either it is suspicious, dangerous behavior or it isn't. I think SA may very well have been involved, but that would be equally likely to be true if he were a suit-wearing, golf-playing social butterfly.
The way I see it, truly suspicious factors are ones which can be used independently to suggest guilt. If you start by assuming things like "Goths are more likely to be guilty" you quickly run into problems. If there are three Goth-types involved in a case, and you knew only one could have possibly done the crime, and all you knew aside from that was that they were Goth...how do you choose the criminal based on that?
You've already decided they're all more likely to be guilty because they are Goths, yet clearly, the majority of these Goths did not commit the crime. So it is too in life -- there's a lot of people with unusual fashion tastes and very very few of them are killers. That means if you find one who happens to be a killer, it's just a coincidence.
Even if it happens to relate to a sick mind, a sick mind can latch onto anything and look like anyone. Again, if it happens to latch onto something that makes you uncomfortable to begin with, you might perceive it as meaningful, but the empirical reality is that it is not.
John Wayne Gacy was very well liked by his neighbors and known as a friendly, kind man. By the time they stopped him, he had so many dead bodies hidden in his home he had run out of places to stash them. For every example of someone who seemed outwardly weird before a crime, there is a counter-example of someone who seemed perfectly normal, which means neither situation is logically more suggestive than the other.