IMO, whether it’s “mental illness,” the bursting of the “entitle mentality” illusion, “narcissism,” “bad parenting,” or any other “cause,” what I find sad and disgusting in all of these cases is, that many of these people choose to share their misery with other innocent people by destroying or trying to them.
If they can’t get help, don’t respond to it, or simply don’t want it, it’s unfortunate that the “miserable” can’t seem to keep their misery to themselves and leave everyone else alone.