Not sure what makes her a "shitty person" in Ep. I and II. The only person she's particularly mean to is her brother, to whom she has a complicate relationship because he's emotionally constipated. Other than that, she's very kind to MOMO for example, tries to understand the Commander before he passes on, slams Allen for his racist beliefs, consoles MOMO multiple times and slams Albedo for being a mass-murder who uses people. Oh I guess she's kinda mean to Allen at times, but it's more like indifference than anything. You are supposed to laugh at those scenes anyway.
Different people react to different situations in different ways. The most tragic human character other than Shion is Jr., who was created as a weapon and ironically had someone who understood him at his side for all his life (Nigredo). Shion was 7 years old when her mother, father and nanny were impaled in front of her eyes. After that she never sorted her issues, not even as an adult. Her fiance dies, she finds out the company she works for is behind the latest catastrophes, she is "forced" to think her father worked for the space terrorists and sacrificed her mother for a while, she's dying and her robot-daughter is killing her and she's forced to revisit her childhood and finds out that she's responsible for the death of billions of people and the end of the universe. (And those are the stuff just on the top of my head, let's not get into Almadel or Ep. 1's fun times into the Encephalon)
Yeah, not shit she's being critical and self-centered, it's the whole point of her character! She's mentally ill and not making correct-life choices.
Even the robot gets it.