Because he'll be too lazy to lazy to listen to the Kah-Gash. Remember it keeps telling him to let it take over? He keeps on refusing and the situations become worse every moment. What if, and here comes the theory, Grubbs thinks at a certain moment: " Damn, it's gone too far, I'll unleash the full Kah-Gash by grabbing Kernel and Bec with me," and he unleashes the Kah-Gash. But the situation is so bad that the only thing the Kah-Gash can do is destroy the universe. So, Grubbs will destroy the world after all. He realises in his last moments what he's actually done and ends his life with the screams of demons and a savage wolven howl.