The existence of the death note proves the existence of the afterlife, and as the one who found it I am clearly the predestined divine ruler of earth.
Using the death note causes ones soul to be irreparably marked and when you die your soul is obliterated (iirc the Shinigami eat it but it’s been a while so idk specifics end result is obliteration)
My death note would be used as a tool of power. First to create the ultimate prisoner’s dilemma. Simply write down the names of your accomplices, and they will be absolved of all crimes (go to heaven) while you will be given a title of nobility in the kingdom (might as well make your life worthwhile since you won’t get the afterlife).
I would even allow the justice system to use it as punishment. Have criminals sentenced to obliteration use it on criminals sentenced to mere death.
Scientists of course would be given a copy. Since it kills the heart, what about pacemakers? There’s a small delay in effect implying that it must travel at some speed, so does relatively matter and in principle could you outrun the effect? Can we draw power from soul obliteration? Can we draw power from the afterlife? Can we bridge into the afterlife and turn souls into power gems??? How does the paper know what language to use? Many tests to run, many sadistic villains with damned souls which I’ll have to run them.
Also since it would inevitably be used on me in some way, or I would inevitably be forced or tricked into using it, I’ll just have to die knowing the memory of tyranny will persist and make it into the afterlife.
Damn. Well that’s why we have a science division, their job is to figure it out. We’ll just have to use some made up religion to quell the masses until they’ve mastered death note theory…