Apatan Hit Man
This is a sniper I created for an adventure around a year ago. The physiology of his species is highly reactive to what the rest of the galaxy considers "Type I" atmosphere, and must wear full-body protective gear in order to survive. Any exposed tissue will begin to boil and fizzle away, and inhaled air has explosive results. This hit man was forced to such an end when the PCs had him cornered for interrogation and he removed his helmet. It wasn't pretty.