These are the endings and the ways to get them in Disgaea: Hour of Darkness.
Main story mode endings:
Best Ending: defeat the game normally without a single ally killed.
Dark Assembly Ending: "persuade by force" bills 50 times, then end the game normally (must have 1 ally killed other wise the best ending will play instead).
Earth Conquered Ending: during the final episode pass the "Human world" bill and defeat all it's stages.
Etna Heroine Ending: kill at least 100 allies before episode 5 stage 4, and when given the option to kill or not to kill Maderas, kill him.
Flonne Tragedy Ending: kill at least 50 allies before episode 3 stage three and when given the option to kill or not to kill Hoggmesier, kill him.
Item God Ending: go to an item world (it's best to go to items that start with eniemes at level 255+ to do this) and kill the Item God, or Item God 2, and end the game normally (but must have at least one ally killed otherwise it will be the "Best ending" instead").
Mid-Boss Ending: loose to Mid-boss during any stage you fight him in.
Normal Ending: defeat the game normally with at least 1 ally killed.
Etna mode endings:
Beauty Tyrant Ending: Defeat Baal in Etna Mode.
Normal Ending: defeat the game normally in Etna Mode.