Latimer claims to be able to read Alfred's mind and he says his brother doesn't have doubt or fears, but nobody is completely free from doubt and fear. Latimer doesn't really know his brother but he thinks he does.
The narrator, Latimer, is unreliable and his words can't be taken as fact. He claims that there is no evil in store for Alfred and if he didn't marry Bertha it was because someone better had come along. But Alfred dies that very day. Evil does befall him and he doesn't get a chance to marry Bertha or refuse her.
"The fear of poison is feeble against the sense of thirst": The need, the want of a relationship overcomes the intuition telling a person that the other is bad for them.
"Bertha that is not your real feeling": Latimer is projecting his own emotions onto Bertha - he doesn't want her to marry someone she doesn't love, and he knows they're going to be married, he wants her to love him!
"The easiest way to deceive a poet is to tell him the truth." She is feeling exactly what she's portraying but Latimer is purposefully misinterpreting her because of what he wants her to feel. Latimer doesn't have supernatural powers but rather an overactive imagination with the ability to project his own emotions and wants onto other people. When Latimer loves Bertha, she's portrayed as an amazing wonderful person; but when he doesn't lover her, she becomes a horrible monster. In both cases, the Bertha he "knows" is a projection, first an idealization and secondly a demonization. But he doesn't actually know her at all.
Does Latimer really love Bertha? No, he loves her for what he thinks she should be, for his inner portrayal, his fantasy, of her.
Did Latimer kill his brother? Did he want to marry Bertha so badly, was him "seeing" her married to him so overwhelming, that he killed his brother? Maybe.
Is judging realism? In order to escape your own pettiness, you judge other people's pettiness. Judgement raises a person above others, so when Latimer talks of Bertha's negativity and Alfred's shallowness, he's really seeing those things in himself but not accepting them so he finds those faults in others.
Latimer realized that he didn't love Bertha after his father died, he didn't need her anymore - nobody to compete with, nobody to prove himself to. The need for the relationship was gone.