In the film, I Am Number Four, every time an alien dies, they turn to dust. The manner of death doesn't seem to matter, as we see this happen to aliens who are stabbed, shot, etc. The film suggests that we're seeing many different species of alien, from many different planets. And yet, any non-human in the movie turns to dust when dead. Is there some explanation for this?
I think BBlake is right. I haven't seen or heard an in-universe reason for it. I think it's a combination of "so there are no bodies for the government to find" and "this makes the aliens look extra alieny".
The Loric do not turn to dust (they did in the movie but that was wrong), only the mogadorians do. This is because they had evolved with rapidly decaying genes, it is briefly touched upon in The Lost Files: Nine's Legacy - Sandor uses the IMog which uses the dna of a mogadorian to track them however Nine mentions that they constantly have to refill it, so to speak, becasue it decays so quickly.