Among the tenants in that big old house were the children Eva took in. Operating on a private scheme of preference and prejudice, she sent off for children she had seen from the balcony or whose circumstances she had heard about . . ." (1921.21)
It's interesting that Eva seems to have such compassion for children who aren't her own. She cares for these kids by choice, and while she is only willing to take in the ones she likes, she still pulls them out of their "circumstances." This from a woman who seems to have only felt burdened by her own children.
Leave him 'lone, I said. Come on, Chicken. Look. I'll help you climb a tree. (1922.49)
Sula is compassionate as a child. When Nel picks on Chicken, Sula stands up for him and seeks to protect him. Compare this to the lack of compassion she has later in life.
Sh. Sh. Don't, don't. You didn't mean it. It ain't your fault. Sh. Sh. Come on, le's go, Sula. (1922.81)
Instead of judging Sula for Chicken's death, Nel tries to comfort her. She is almost a maternal figure here: she feels sad for her friend and wants to make her feel better and calm her down.
It hit her like a sledge hammer, and it was then that she knew what to feel. A liquid trail of hate flooded her chest. Knowing that she would hate him long and well filled her with pleasant anticipation. (1921.18-4.19)
When BoyBoy visits Eva, she's not sure how she will react. When he leaves with his other woman, she knows that what she will never feel is forgiveness, and this pleases her. She will hang on to her hatred of him as a way to comfort herself. Forgiveness simply wouldn't sustain her the way hatred will.
Eva looked at Sula pretty much the same way she had looked at BoyBoy that time when he returned after he'd left her without a dime or a prospect of one. (1937.8)
Eva's unwillingness to forgive extends to her granddaughter. She equates Sula's long absence with her husband's abandonment, even though the two are completely different. But instead of the comfort she gets from not forgiving BoyBoy, Eva just seems bitter here. Her anger is misdirected in that she doesn't draw any distinctions between the people she feels have wronged her.
I heard you was sick. Anything I can do for you? (1940.1)
These are the first words Nel utters to Sula after the affair with Jude. Although she may be saying them out of a sense of duty, she still shows compassion for her. She doesn't just ask her how she's doing; she asks her what she can to do help her. And while she may not have forgiven Sula, her willingness to lend her a hand shows that she still has compassion for her.
Embarrassed, irritable and a little ashamed, Nel rose to go. "Goodbye, Sula. I don't reckon I'll be back." (1940.79)
This is Nel's final opportunity to forgive Sula while she's still alive, and it doesn't happen. She has no intention of seeing Sula again, and while she may wonder about her and maybe even feel sorry for her, she won't forgive her.
Who was it that had promised her a sleep of water always? (1940.92)
Sula is comforted by remembering her encounter with Shadrack after Chicken's accident. His compassion for her is profound enough to soothe her in the painful throes of death.
The death of Sula Peace was the best news folks up in the Bottom had had since the promise of work at the tunnel. (1941.1)
There is an utter lack of compassion for Sula from the residents of the Bottom. They feel relieved and hopeful now that she's gone, and their response to her death is callous. Although she may have caused a lot of pain and created a lot of trouble, Sula is still a person. But the Bottom sees her as something to get out of the way.
Sadly, heavily, Nel left the colored part of the cemetery. (1965.66)
After visiting Sula's grave and remembering how few people actually concerned themselves about her death, Nel experiences true compassion for her friend. This precipitates her realization at the end of the novel.