Chief Justice of the U.S. Supreme Court, John Roberts, examples here an extremely common body language tell - and a very valuable one in the negotiation process (particularly for a Judge, he has a horrible poker face). Whatever verbal message that is conveyed when/just before/after a person displays such a "Bitter Smile" (Ekman) - it is an absolute that their true emotions are being held back - very often in flavor, and always in amplitude.
Whenever the verbal and nonverbal messages are in disparity, the nonverbal is representative of what is truly being felt and thought. This particular bitter-regret "smile" guarantees some disparity.
Believe it or not, we all make this bitter smile much more often than we think. The vast majority of the times, people think they are making a slight, polite smile (a "social smile") - when in actuality our mouths are concave down! (and our eyes are very far from "smiling eyes" - see Secret # 353: Brad Pitt's Smiling Eyes - Sincere and Insincere Smiles). This is yet another profound example of just how much our "emotional-proprioception" is desensitized during moments of negative emotions. In other words, virtually everyone is extremely unaware of just how much anger, sadness, fear, anxiety, vulnerability, etc. are displayed on their faces and bodies. This is very true aspect of human behavior, regardless of whether it's in retrospection or in real-time.
Chief Justice Roberts is rumored to have a genius level I.Q. But remember, I.Q. is an abbreviation for "Intelligence Quotient" - intellectual intelligence, not emotional intelligence or emotional awareness. Ironic to some, those with a lot of formal education, on-average, have considerably less emotional awareness than those with only a high school education. We tend to ride more on our J.D., M.D., Ph.D, MBA, etc. and less on developing and maintaining our interpersonal skills. This is a pit-fall for which many (even most) attorneys, physicians, politicians and C-level executives suffer - they are largely in polite denial. Indeed, the average physician is superior at diagnosis, surgery and running a small business when they're 37, 47 or 57 - but their interpersonal skills very often were better when they were 27. The emperor is naked.