A tsunami takes life, cuts it like a sword through thread, and leaves everything behind completely pure. Everything is dead and gone, wiped clean off the face of the earth, thanks to the tsunami. It's back to the way it was before humans were here, it's completely pure.
After the tsunami pulls away, everything left is pure white. You hate this, so you run over and squirt ketsup (けつ) all over the place to make it not so pure.