Being unafraid of death wouldn't mean you have a disorder.I'm not sure that some one who has autism really knows it.When people are young they tend to feel invincible like nothing could hurt them and could therefore not be afraid of death.On the other hand you could be older and just accept that one day you will die and therefore choose not to be afraid of death. I on the same token am 26 and I am really scared of dying. I am not ready to accept that I will one day I will need to leave this planet. I have not done everything I want to do and it makes me nervous. It just goes to show that you need to live everyday to the fullest and do everything you feel passionate about.
Someone who doesn't know fear - unafraid of death or being harmed may indeed be autistic. They might also suffer from bipolar disorder - in a manic (happy) state - people don't have realistic fears and tend to be reckless. Also, sociopaths sometimes have little regard for natural consequences of behavior - such as being harmed from being impulsive and dangerous. Some people have brain damage that keeps them from functioning normally and having normal fears. In a nutshell - there are a number of things that could cause someone to feel this way.