My biggest disappointment is not that this wasn’t the same story as the book. It’s not. It is a completely new story that has absolutely no bearing on the original material.
The real disappointment is that the main character, John Kelly, is really not portrayed as the character that makes him such a force in the books. John Kelly/Clark was a cool, calculating, planner, that was controlled and planned three steps ahead. He wondered at times why he felt no remorse, and contemplated on it and that he knew right from wrong. He was discreet, and always worried about collateral damage.
The character in this move adaptation is reckless and acts too much on the impulse. It captures the spirit of John Kelly getting lucky sometimes. But the movie just tells too much on Kelly being good at combat, and less at how cool and calculating he was.
This movie is a "classic, near-cliche case of keeping the book's title," two characters (Kelly & Ritter) and nothing else. The elements could have been reworked as a period film in the mid-2000s with Kelly as an Iraq war vet. Without Remorse was set in the 70s and Rainbow Six was contemporary, Kelly-now-Clark middle aged and no longer in the field (which I guess went out the window when they cast Jordan).
Put it this way, Without Remorse is evident of that but despite Jordan's best efforts, the film falls flat due to a story that is flat and characters that are undeveloped. Overall, its a decent action film, worth watching it, I hope the sequel they are definitely setting up takes some of the critiques aimed at this film and listens, because otherwise it would just be a disservice to the talents involved and the great characters we could be seeing portrayed.