I stated in my review of the game that it focuses on positive themes despite it's dark premise. Love and Justice are one of the biggest themes.
"Eternal Love ♥"
Love is one of the recurring themes of Disgaea series. Romance being a main driver of Disgaea 2 (the first game I played in Series.) I was surprised to find out how love, another kind of it, plays a bigger role in the first game.
but there are some spoilers. BE WARNED
The bulk of Disgaea story happens in the netherworld, a place where Demons live, Laharl the main character is the son of the late Overlord and through the story he fights to become the next overlord. His adventure encounters him with Rivals Humans and Angels.
Flonne The Love Freak
Early in game Laharl meets Flonne an angel sent to kill his father, but when she heard of her target death and his son's apparent apathy toward that, she made it her mission to see if Demons are really incapable of Love.
"If that's true, I'll carry my mission on a new target."
Love is the definition of Flonne, she always spoke idealistically and shows kindness even to enemies. That might have worked against her as helping an enemy isn't something one should do.
In the Netherworld Flonne encountered a huge culture shock in the netherworld, where the norm is against everything that she was taught, but instead of trying to reject those who are different she tried to understand their reasons.
Kindness Is Love
Laharl while too harsh and proud, is not merciless and can show kindness like when he spared an Enemy after knowing he had a family. He also forgave betrayal more than once after knowing their reasons.
He changes through the story from a arrogant brat who only cares about himself to a proud guy who has a bit of kindness, mostly due to Flonne.
The Red Moon
There's also the familial love, while Laharl claims to hate his father (depending on my interpretation he seems to be lying.) He loved his mother before she died, the one love he didn't (proudly) deny.
Love = Sacrifice
Part of the game explores the effects his mother's death had on him, his annoyed relationship with Flonne stems from how similar she is to her. And the most moving scene in the whole game is when Laharl finally comes to terms with his mother's death.
Understanding The Other Perspective
Demons are creatures associated with Evil, while Angels are the embodiment of Good.
Disgaea is one of the best examples of Good vs Evil stories. Flonne is so good angel, so filled with love that she believed the quote above is wrong. That statement was silently questioned all through the game.
A great way to teach Open-mindedness
While Laharl has many bad traits, he is just selfish and never really Evil. Etna doesn't care about good or evil but admired the goodness of the late Overlord (Laharl father the benevolent king.) That and the most Evil character in the game is an Angel (Even him has a good intentions.)
Open-mindedness is the whole point of the game. "Don't judge people by their labels, and don't consider yourself inherently righteous." because the moment you do, you'll do evil in the name of justice. Prejudice is where all evil comes from.
Conclusion
No Sympathy At All?
Disgaea: hour of Darkness (and by extension the PC version I played,) deals beautifully with the theme of Love, kindness and open-mindedness. Having a comedy game let me have of all these profound thoughts, I can only recommend this experience.