Although you are more than welcome to attend a horror convention in street clothes, you are also very welcome to pretend it's Halloween and arrive in costume. Obviously, since this is a horror convention, a scary costume will work much better than something less frightening.
Conferences usually lean towards business casual attire, but the exact dress code can depend on the specific industry or the nature of the event. A tech conference may endorse a more relaxed dress code, while more traditional sectors might require formal attire.