Why are People so Cruel?

Something I found ironically funny is the way the main character in the book The Shawl  by Cynthia Ozick, who is crazy in the book, finds her niece, Stella, to be so cruel. What makes a person cruel? I know everyone has different opinions, but is how Stella treating her aunt considered to be cruel or is she simply giving her elderly aunt tough love?
Throughout Rosa Lublin, the main character's, time in the concentration camps in Poland, she also took care of her 14 year old niece, Stella. From the very beginning, Ozick writes, "Stella, cold, cold, the coldness of hell." This opening line unveils a sense of intrigue to the reader and after careful analysis, I realize this was Rosa's way of looking back to describe her cold and selfish niece who wanted to be cared for at a time when she was split by both her parents and was in the care of an older aunt with a secret child of her own. I partially think for Stella being so young it was hard for her to handle the loss of her parents and her sanctity. As Ozick mentions on page 6, "And afterward she was always cold, always. The cold went into her heart."
Later on, there are other forms of Stella being cruel. This is an excerpt from a letter from Stella to her aunt on page 31. Ozick writes, "You make yourself crazy, everyone thinks you're a crazy woman. You'll open the box and take it out and cry, and you'll kiss it like a crazy person. Rosa, it's time you have a life." This is upon her returning the shawl from Rosa's dead baby to her. This is cruel, in my opinion. But maybe there is a reason for Stella's unusually hostile behavior. Perhaps she wished her aunt took better care and paid more attention to her as a growing girl during those tormenting years. She never had her parents. Maybe Stella is hostile towards her aunt for harboring a loss that was not really her fault. I think deep down she wants her aunt to move on, yet she (Rosa) takes Stella being cruel and working against the loss of her child.
A lot of readers can view this as cruel, but part of me sees it as a natural reaction as the result of fear, depression, and neglect.