In the old lore, it used to be that old gods shouldn't be killed because they were so intertwined with the planet, that killing them would cause massive devastation to the planet.
Lo and behold, we have killed (or at the very least, killed his body) the ostensibly strongest old god, with no apparent old changing consequences. How about we go back and finish off his two weaker cousins? We're also far stronger. Should be a piece of cake, right?