
Why does she wanna leave? Is she getting bullied?

Yeah, she is. She was with the first group of outside people the MC made contact with. She was granted the ability of poisonous gas by MC's ability but we haven't seen her use it yet. The mc also mentioned being worried about her using her ability that one time when the group was in a confined space because he didn't know how her ability operated yet.
This is insane. She went through grief and was going to put him through that too knowing how it feels. Oh well, I, glad she at least acknowledged that she shouldn’t have done that. I do get wanting to die and join her loved ones though. This truly is heartbreaking.
When will things get better? Will it ever get better?
In sia's defense, she was grieving after loosing her mother, unborn brother and older brother. In that state with the physical mind of a child she couldn't think of anything bit herself and her grief ╥﹏╥.
In a way she was kinda suicidal with it, she knew it was poisoned but still kept on eating eat even while coughing up blood.