The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at the Trope Launch Pad.
Edit: Nevermind, that's disambiguated, but maybe it can lead you in the right direction?
Edited by Serilly LP Deprecation Cleanup | Ask me about SMP LiveFor this example, I think you want something in the vein of Crime After Crime (if this is a matter of killing everybody that notices they're killing everybody) or Mike Nelson, Destroyer of Worlds (They only mean to kill their target, but their incompetence means they end up killing everyone around them in the process). Could also be a Misanthrope Supreme if they're doing a flanderized roleplay of the character as killing everyone that slightly inconveniences them.
In general, though, Tropes Are Flexible- you can ignore one aspect of the trope if they fulfill all the other important ones. The Toaster from Fallout: New Vegas is a fantastic example of an omnicidal manic without the power to immediately act on his desires: a cackling Axe-Crazy AI who loudly and frequently expresses his desire to incinerate what is left of the post-apocalyptic world... currently serving as the OS for a smart toaster, and channeling his rage into the destruction of other small appliances (which lets you turn Vendor Trash into crafting materials) and vicarious destruction by helping the player craft superheated weapons from a certain form of phlebotinum.
Edited by Scorpion451

What do you call a character who wants to kill everyone they see, but doesn't necessarily have the power or intention to destroy their entire world like an Omnicidal Maniac?
- Robbaz: Played for Laughs. In his Hitman videos, Robert is not content with just killing his assassin character's targets - he usually winds up killing every NPC in the game that he comes across.
Edited by DrNoPuma