Nico was older now, 16 years old was how old. Not much, but the age changed everything. The insane gene hades kids have had finally appeared in Nico. He went....... A little..........Insane. After his trip to Tartarus and Wars he has been in have finally caught up to him, he went absolutely insane.
Most hades kids have the insane gene, although a few very rare times are that they don't and are sane, like Hazel.
But after Nico's gene makes a show up, he has a thirst in killing people. He had also found a way to be immortal, without being a god. Thus causing Nico's banishment, no longer allowed to enter Camp grounds because of his insanity.