Hero Killer is a particularly fearsome type of villain who has successfully killed at least one protagonist in a story, whether they killed a supporting protagonist or, in very rare cases, the main protagonist. Examples include:

• Dream killing Ghostbur, as well as temporarily killing TommyInnit.
• Francis having Desti killed by False Sephiroth.
• Bowser killing the Mario Brothers.
• Hit The Target killing several of Stampy’s Dogs and one of his helpers.
• Blackrabbit murdering her father Fritz.
• Michael killing Scott Cawthon and assimilating his body.
• Mikado Sannoji killing Teruya Ōtori.
• Ultimate Protobot 10000 erasing Shadow Guy from existence.

This can also count villains who caused the death of a hero, without actually physically killing them themselves (e.g. Niles, who caused Axol's death. and Lieutenant Obsidian, who threw Jade into a flooded ravine, causing Ruby Diorite to sacrifice herself to save Jade, making her responsible for Ruby’s death).

This type of villain usually invokes a grim reminder that anyone can die if the work is dark. If the work was fairly light-hearted before this villain appeared, then these villains are often a Game Changer.