Good reasons:
1. It advances the plot
2. It fulfils the doomed character's personal goal
3. It motivates other characters
4. It's a fitting recompense for the character's actions up to this point
5. It emphasizes the theme
7. It creates realism within the story world
8. It removes an extraneous character
Bad Reasons:
1. Shocking readers just for the sake of shocking them
2. Making the readers sad just for the sake of making them sad
3. Removing an extraneous character (Yes, this is also a good reason. But double-check. If the character is extraneous, first verify he really belongs in the story in the first place.)
