Perhaps the key incident is the death of Curley’s wife
The scene describes her seductive nature clearly, with a focus on her clothing.
She talks to Lennie like one would talk to a child. It becomes apparent that they are both isolated, lonely characters.
She tells Lennie her secrets.
Lennie strokes her hair, but gets confused and holds on.
His fear and confusion cause him to kill her, just as he has killed the mouse and the pup before (these act as events that foreshadow this main incident).
He tries to cover up her body in they hay, like a guilty child.
The whole world goes silent and there is a threatening atmosphere from this point on.
George is sad at the murder and knows what he has to do.
The ending is sad, George kills Lennie to stop further accidents happening.
Lennie acts like a child being told off, he doesn’t see quite how serious it is.
George finds it difficult to kill Lennie and tells him of his dream even at the end, to soften the blow.
The dream is effectively shattered at the end of the novel.