To make the movie work the Master Chief will have to be a character that people can/will feel empathy towards. This will mean that MC will have a personality, a background, a face (probably), and also probably a love interest. To make a movie that appeals to an audience outside your fanbase this is essential.
This will probably totally ruin the story and characters, and is why I personally don't want a Marathon movie made. But if Bungie want a Halo movie then, go for it. Most people tend to forget that Bungie is a company, and like all companies, it's primary goal is to make money (not world domination sadly).
Also how many computer game/movie transformations have worked? I can't think of one. Have you guys heard of the Doom movie? It is about a SWAT team in an abandoned space station where a virus mutates people into monsters. How can you possibly stuff up a movie plot that can be described in two sentences?
"I'ts on mars! Oh shit, hell!"
But hey, mutating virus it is.
