Injuries that leave scars can be the result of self-defense, so I don't think that would necessarily help make the victim look less sympathetic. If you want to make the victim look less sympathetic, you might need a bit more space to properly show it outside of when they're trying not to die.
My first thought, without knowing any context, would be to focus on Protag's emotions surrounding the act. You could show him justifying the murder in his thoughts without revealing anything about why he has those justifications, for instance, or focus on the ways he deals with the cognitive dissonance of killing someone, such as not looking at the victim's face or blocking out their voice. You could focus on how they're not deriving any enjoyment from the activity, in addition to the self-justifications. It depends on what you're going for, but human emotions and a visible motivation, even if the nature of that motivation isn't clear, are half the battle of getting an audience invested in a character and what happens to them. Portraying them sympathetically, however, might be harder here. Note that wanting to know what happens to a character and sympathizing with a character are not necessarily the same thing, and having a protagonist that people are "merely" invested in and don't necessarily identify with or like can sometimes be more effective than a sympathetic one. So that may be something to think about.
As for revealing the motive, I think the Motive Rant might be the lesser of two evils here. I find that several kinds of flashbacks in text-only media can break immersion and force the reader to have to reorient themselves in the flashback, and then reorient themselves a second time once the flashback's over. Alternatively, you could have another character reveal the motive, for instance.
"Jack, you have debauched my sloth."
My story opens with the protagonist killing someone and then disposing of the body. I'm not going to reveal the motive behind the murder until later in the story, with it being something of a twist. I'm worried if my protagonist can still be sympathetic if the audience's first impression is of someone who murdered someone with no reason. Note, this isn't an anomaly for this character. They're willing to kill people if push comes to shove. I'm also wondering what's the best way to reveal the motive. Would it resonate better with the reader to reveal it in flashback, or have my protagonist go on a motive rant?
Edit: One idea I have is to have the murder victim give the protagonist a scar. This isn't the only reason that the protagonist wants to kill them, but it will act as a physical symbol as to how bad they were.
Edited by RustBeard on Aug 31st 2018 at 5:21:37 AM