A Spreading Stain.

That moment when something fluid and dark hits something light.  It begins to spread, and consume.  The inky creep of dark things against light. The middle is still thick, viscous, glistening.  But the edges are creeping- they are spreading out from the center, like little liquid soldiers on their little liquid missions. 

That is the progression of a bad mood- but not just any bad mood, the kind of bad mood that infects everything, and everyone around you.  It's the creep of toxic foul thoughts that begins in your (hopefully) irrational brain, overwhelms your rational brain, and then leeches into your entire being.  It is the spreading stain.