All I remember is a scene where someone is running away (towards camera) down a trench or a stream. There is some sort of disaster/monster they are running away from (virus? nuclear? zombies?) and they fall down (again towards camera) with a lot of extreme gore to the face, possibly exploding head or melting eyes.

The style is anime, like the kill bill anime sequence or the second renaissance segment of animatrix.

I seem to think this is a flashback or story in a live action film. They might even lampshade that it’s too gory to show as live action?