Why? What idea or practice is this grim reaper rebelling against? Is it about the job? Is it personal? Why now? What has changed to prompt it? Why her or him?
Has a reaper rebelled before? If not, why? If so, why? What was the outcome then? How will it affect how the death world responds to this reaper?
What would "going rouge" mean for a reaper? Typically reapers collect the souls of the dead. Is the reaper not collecting? Not delivering?
Is that all reapers do in the world you're imagining? Do they have other tasks? Has something become more important to this reaper than collecting souls?
Are deaths pure chance or does someone or something determine how long someone's life is? Is that process part of the reaper's choice to go rogue?