I think it's primarily the death of the murderer -- the protagonist does not make a decision on-page about what he's going to do wrt his revelation until after we've learned about the killer's self-inflicted 'accident' -- so there can be no further death at his hands, combined with the social stigma the reveal would inflict on the killer's wife and child. Which I get! I'd just feel better about that calculus if, for example, we'd ever seen the grieving parents of the murder victim on the page or gotten any sense that they factored into the equation.