Ending twist **Spoilers**
I was just commenting on "The Nameless" message board about the very same thing that bugged me with this movie too. It seems they go for a twist ending that just ruins everything that went before in the movie.
If you committed a crime, would you hire your best friend--who by the way is eager to please you because you so-called save his life--to investigate it? Wouldn't you hire some inept flunky just for show?
In my opinion, there is NO plausible explanation for why Carlos would have hired Xavier to investigate the case. But I would love to read any opposing opinions.