In "A Rose for Emily," Emily cannot be convicted of either one, really. She's dead. That's the whole point of the story. The town is clueless. It's the worst piece of detective fiction in the history of literature.
Obviously, she might have been convicted of any number of crimes if the townspeople would not have institutionalized her (treated her as such a monument and pillar of the community). They can't even get her to pay her taxes. How are they going to have the guts to charge her with murder or, worse, insanity? She doesn't even need a lawyer. The town is too scared to even enter the house, let alone deliver a subpoena.
The signs of premeditation are there: she buys rat poisoning, Arsenic. The town, though, mistakenly thinks she's going to kill herself because no one will marry her. What?! They honestly thought and even condoned suicide over being an old maid. What fools!
And there's evidence: a certain fat decomposing body in the bedroom. Hello?
After she buys the arsenic, she is seen with Homer Baron the very next day. Even though he is a Yankee and openly gay, they also condone the marriage. Fools again! They don't see signs of murder or poisoning even after he disappears and the smell of a dead body fills the neighborhood.
Putting Emily on trail for pre-meditated murder is like putting Oedipus on trial for being blind: it's rather obvious. CSI is not needed here.
The key to determining a premeditation is the existence of a time line or a behavioral pattern that lead up to the commission of the crime. In her case, there is indeed a trail leading up to Homer, beginning with her trip to the apothecary to get arsenic, and his eventual disappearance.
An insanity plea is often entered when it is proven that the person who committed the crime could not define right from wrong due to mental illness. The problem with this plea is that it also includes "temporary insanity" which could have been entered for Emily considering that she does not have a previous criminal record.
Ultimately, the finding of Homer's corpse would make anyone think that she was in fact insane. However, this form of fascination and inability to let go manifested itself after the killing. Emily would have been declared insane if she had killed Homer IN ORDER TO sleep with his corpse. That would be insane. But, instead, she killed Homer with the purpose of ending his life. That is what amounts to premeditation.
It would be hard from the evidence in the story to actually prove anything. But looking at that evidence, here's what I would say.
First, I would say that you do not typically accidentally poison someone with rat poison. She clearly went to the store specifically to buy poison. And she would not answer the clerk when he asked her what it was for. That implies that she knew what she was going to do with it.
If she knew what she was going to do with it and she knew that she should not tell people, then she knew that killing Homer was wrong. So that proves she's not so insane as to not know right from wrong.