a melodramatic way of signalling that what follows will be a suicide note
I doubt that most suicides would actually start their note in that way - if any.
The world is not always cruel. The degree of "cruelty" a person subjectively experiences might not be due to flaws of temperament, character, or habitual negative thinking; he or she could be the victim of entrapment by a psychopath or other circumstances beyond their control.
Maybe the world could be perceived as unremittingly cruel by someone with disordered thinking and long-term clinical depression, but even then, I doubt they would start their note with those words.
