What are some character's traits of Bruno in the book boys in striped pajamas that cause his downfall??

I would describe Bruno as being a bright, curious and empathetic boy. He does not take things at face value. He sees Shmuel not as a prisoner but as a caring boy much like himself. Even in the face of death, Bruno holds his friends hand,

"...Despite the chaos that followed, Bruno found that he was still holding Shmuel's hand in his own and nothing in the world would have persuaded him to let go".

It is this intrinsic empathy with people less fortunate than him that leads to his downfall. Bruno stands by his friend even in the face of mortal danger.