Of course, it is true that the death of Curley's wife led to Lennie's death.
However, I think the intent of this question is that the dream that George, Lennie and Candy had cherished and seemed so close to realizing also died.
George and Lennie had long dreamt of owning a 10-acre farm where they could "live on the fatta the lan'," raise rabbits for Lennie to tend, settle comfortably and have control over their own lives. Candy had recently become a part of this vision and provided the money that would make it possible. His desire was mainly to have a dignified and kind way to live out his years once he was unable to work.
In the barn on that fateful Sunday, when Candy shows George the body of Curley's wife, he says:
'You an' me can get that little place, can't we, George? You an' me can go there an' live nice, can't we, George? Can't we?'
Before George answered, Candy dropped his head and looked down at the hay. He knew.
George said softly, 'I think I knowed from the very first. I think I knowed we'd never do her. He usta like to hear about it so much I got to thinking maybe we would.'
'Then -- it's all off?' Candy asked sulkily.
George didn't answer his question. George said, 'I'll work my month an' I'll take my fifty bucks an' I'll stay all night in some lousy cat house. Or I'll set in some poolroom till ever'body goes home. An' then I'll come back an' work another month an' I'll have fifty bucks more.
Curley’s wife and her dog are the only two that died in the barn on that day but it was because of the actions that Lennie took in the barn that caused him to die later on that day. Therefore, you could say that Lennie metaphorically died in the barn as well. After the events of Curley’s wife’s murder, Lennie, under the advice of George, runs away to the stream that is described in Chapter 1. It is here that George sympathetically takes Lennie’s life in order to save him from the brutal murder that may have taken place if Curley and the others had found him before George did.