There are some forces in this world that are better left untouched.
The Deep One is an enigma of the Avatar world. Some may call it a spirit, but unlike the vast majority of spirits, it lacks any sort of good/evil dichotomous nature. It isn't good or evil, but it is entirely devoted to a single concept: chaos. Chaos is its goal, its nature, its entire being. It cares not for sins or the whims of mortals, nor for the fate of the spirit world and the desired balance therein. All it cares about is bringing entropy wherever it can, especially the mortal plane. For it is here that order truly reigns, especially with the defeat of Vaatu, the one spirit The Deep One entrusted with its sacred mission. Instead of reaving the planet as it was told, he sought to make an empire for himself. The only chaos he wrought was when it was convenient for him, against the Deep One's wishes.
Now it entrusts the mission to mortals. For it is they, above all beings, who delude themselves into thinking that appealing to the end of things will allow them to survive. More importantly, however, they have the power to bring The Deep One into the waking world, but not so much that they will defy it.