Guysss! I finally watched the scene and I think I figured out an interpretation that might work! I haven't written this down yet, so bear with me as I attempt to explain...
OK, I believe that there are two distinct reasons Sark went to see Lauren's body with Vaughn, and displayed the emotion he did in front of him, as well as forced Vaughn to look at Lauren's body. The first I believe people have previously touched on: to get Vaughn to have to relive the past, thus weakening him in a way, making him again a slave to his emotions (okay strong wording, but you get the idea...)... Basically, to undercut Vaughn somehow. To make himself look like a better person than Vaughn, even. To push Vaughn's buttons. However you want to say it.
Second, and this is what I think is most telling... Sark had lots of time to rethink his life in jail. He says this to Sloane. He says he is a changed man. When we see Sark break down at the sight of Lauren, we think it is because he loved her. Notice, however, that Sark does not say "I loved her." Though he calls Lauren (in 'Echoes') "the woman I loved," what he says in 'Man of His Word' is this: "Lauren loved me." (He may have used the pronoun rather than the proper noun, but you get the idea.) What he wants -- and the amount of time he spent
alone in jail has certainly added to the intensity of this desire -- is to be
loved by someone. His father never loved him, remember. We don't know anything about his mother. But we know that his father's lack of affection, in the end, drove Sark to kill him. He even refers to this in S2 (Caplan-related ep; can't remember the name...): "What I want is that which I never had." THUS, we can be allowed to conclude that Sark may even be deluding himself into thinking that what he and Lauren had was a healthy relationship. And he may even be deluding himself into thinking that Lauren loved him (no evidence of Lauren's true affection for him is revealed in S3, and like
sugababyboo said, all of the evidence points toward that they would have killed each other if their individual lives or success had depended on it). The thing about being all alone in a cell for a year or so is that it messes with your head. The emotion Sark showed in the scene in MOHW could have come from a completely fabricated feeling he had manufactured for himself to feel loved when he was at his most unloved.
That, my friends, is my interpretation, not very well stated, but I tried... Hope you like!