I think her circumstances and the racist times, etc. may help us understand what she did. I don't think killing is ok, I just think you can understand what she did when you take it in context of what life in the Bottom was like during the 1920s.
While Eva may try to justify her actions, and while we may even kind of see her point, I think that her actions are abhorrent. Who was she to decide that Plum's life wasn't worth living anymore or that there was no hope for her drug addicted son? She may "have her reasons" but that doesn't make the reasons valid enough to excuse first degree murder -- or even a lesser charge of manslaughter. Lots of people do bad things because they feel justified. It doesn't mean they are justified. This scene of the novel is so rich with emotion -- it is a great topic to explore!
There seems to be a certain moral ambiguity about this act that you yourself have highlighted. On the one hand, killing is never a "good" idea, but as you point out, the contextual considerations could lead us to agree with Eva when she says that she killed her son out of love. Morrison paints a terrible picture of what life was like in Bottom, ironically named, and we are forced to ask ourselves hard questions about the extent of love and how love is defined in such situations.
In the novel, Eva attempts to justify her actions when her daughter Hannah questions her about the killing. Eva claims that killing Plum was the only way to set him free. She argues that because Plum was so dependent on her, he had no life or options for himself. Hannah does not think that Eva loved them as children; however, Eva says that the sacrifices that she has had to make over the years prove that she loves her children. So if "justify" means to have a clear reason for acting in a particular way, then Eva's actions were "justified" according to what she reveals to Hannah. Maybe one could argue that killing her son was not the best decision, but this would depend on one's moral values.
I do not believe it was needed to kill Plum since there was a chance that it was still possible to save Plum from further damage and help him in some way or another. However, her reasoning is understandable because her own child is torn and battered from the war and with an addiction, and Eva believes it is better to kill her child than to let him suffer.
I understand that plum's death was almost in a sense a way for him to return to his mothers's womb and be with her more, but i dont understand why plum's death is okay, but sula's was not, from Eva's standpoint....