I've seen people blame Percy for Bianca's death. I've seen people hate on Nico for how he reacted and the things he did in TLO. I've seen people hate on Percy for how he reacted to the whole thing in the Underworld.
Percy, fourteen years old, saw this child, and promised to do his best to keep his sister safe. He never promised to bring her back alive. Only to do his best. Percy did not put a bow and arrow in her hand, Percy did not choose her to go on the quest. Percy came up with a plan to defeat Talos, a plan he would follow through himself, but Bianca insisted she do it, because it was her fault Talos was even attacking them (make no mistake, I am not blaming Bianca for anything, either. This was a bunch of children having to choose between bad and worse).
Percy, fourteen years old, just had to tell a kid his sister, his only sister, is dead. Percy, just back from a quest where he saw two people die, really die, for the first time, is given the responsibility of breaking it to Nico that his only family is dead and gone. They had a kid tell another kid about the other kid now having no sister. Not Chiron, the only real adult, not one of the older counselors who've probably had to do this before. No, they had someone who is still reeling from his own grief break the news.
Percy, fifteen almost sixteen, on the brink of a war he's convinced he's going to die in, just got betrayed. Percy, who is under the constant stress of making a choice that will either lead to destruction or safety, just got betrayed. He is not in a state of mind to consider the variables, to consider what Nico's thought process was. He is a child with the weight of the world on his shoulders (pun not intended), all he sees now is betrayal.
Nico, ten (eleven, twelve? The timeline is weird) years old, made Percy promise that he'd try his best to bring his sister back. To Nico, that's a guarantee that Bianca is coming back, because this amazing, strong, boy already saved them once. He cannot conceive that Percy's best may not be enough.
Nico, ten years old, just found out his sister is dead. Percy (amazing, beautiful, strong, perfect), lied to him. Percy got his sister killed. Percy promised he would bring her back, and then he didn't. It's all Percy's fault, and Nico hates hates hates him (he tells himself he hates Percy, because if he doesn’t then what kind of brother is he? If he doesn't hate Percy with all the hate he has, then is he a good brother to Bianca? Bianca, who left him the first chance she got, who abandoned him.)
Nico, eleven (twelve? thirteen?) years old, just got promised information about his mother. He just has to bring Percy Jackson to the Underworld. He already wants Percy to come down and get Achillies curse, this will just be a detour. But then his father locks Percy away and Nico exhausts himself putting all the guards to sleep so he can get Percy free. But when he does all he gets is a sword to the throat, and later Percy telling him to stay in the Underworld to convince his father to join the war. A dismissal disguised as a war strategy.
And Bianca. Bianca was twelve, and she had to take care of her brother. Bianca was twelve, and she was given the responsibility to be a parent. Bianca wanted to be a child, not a parent. So no matter how much she loved her brother, no matter how much she wants him to be safe and sound, she joins the Hunters of Artemis. She can finally be free, and she takes the chance no matter how much it pains her to see her brother draw away. So she picks up a little gift for him, on her quest, the only figurine he does not have. But then the little gift gets them attacked, and, ultimately, killed.
These are children. These children, presented with circumstances and choices and challenges an adult would break under, are not stable enough to consider every thought, every variable, every circumstance that surrounded every decision made by others. They're under constant stress and pressure and they cannot do this in a 'sensible' way because there are things happening to and around them and they are not stable. Especially at those specific moments.
