In John Steinbeck's novel Of Mice and Men, the character of Lennie is a man whose physical strength and size are disproportionate to his mental abilities and maturity level.
Lennie's mental challenges make him prone to be victimized by others, to be bullied, or simply to be treated like a lesser person. This is the case when Lennie is visited by Curley's wife as part of one her many improper and imprudent visits to the field hands during times where her husband is not around.
Curley's wife is known to be troublesome. The field hands are quite aware of her penchant for looking for men and for dressing seductively. Therefore, this behavior is what ultimately would render her responsible for both her own death as well as that of Lennie's.
The entire incident begins when Curley's wife is "on the prowl" around the men's barracks. We know that she wishes to get Lennie's attention. After all, she has already noticed how corpulent and strong he seems to be and how her husband seems plain in comparison. When she finally gets to Lennie, she catches on quick with the fact that he is mentally slow, hence, she uses Lennie's childish behavior to further seduce him.
When Lennie expresses to her that he likes small animals and soft things Curley's wife, in her incessant quest for male attention, invites Lennie to touch her hair. However, Lennie's almost-supernatural strength ends up pulling Curley's wife's hair too hard and, when she begins to scream in fear, he covers her mouth and accidentally breaks her neck while trying to silence her. Lennie could not have helped himself because it is not malice that made him do it, but his natural clumsiness.
However, Curley's wife was more mentally capable than Lennie and she already knew that she was not supposed to be visiting the field hands in the first place. She should have known that she was putting herself in a dangerous situation anyway by exposing herself to the men the way that she did. Moreover, she knew how brash and violent her husband could be. She should have known quite well that if she got caught with Lennie, Lennie would have been in deep trouble.
Therefore, Curley's wife's flirtatious ways and her need for the attention of men are the factors that render her responsible for Lennie's death. After all, when her body is found and Curley forms a lynch mob to kill Lennie, the only mercy that Lennie gets is that it is George who performs the act of shooting Lennie at close range. Otherwise, Curley would have given Lennie a much more torturous and morbid form of death.