Euphemism for Semen, cum, jizz, ejaculate, spunk, scum, baby batter, etc.
If you're in bad mood and jerk off. You feel better. Because the "Liquid Evil" is out of you.
If you're receiving felatio, and you ejaculate on to the girl's (or guy's) face without warning. They get angry, because they got "Liquid Evil" on them.