Unrestricted violence

Implied: (enforceable)
* Neutral NPCs

Related:
* Friendly fire

Generally any games where the player can harm civilians and others who are completely outside the depicted conflict itself. No immortal characters who can't be harmed, no forced weapons free zones, no automatic non-hostile recognition (turning off attacks when facing them) and so forth.

Only for games where actual third parties exist.

Some games may even go out of their way to throw these third parties in the line of fire intentionally (such as in Carmageddon series).

This also implies that there's no unrealistic penalty on doing any harm to anyone outside the conflict.