Merriam-Webster defines the word “castrate” not just in the obvious sense of the removal of the testes or ovaries, but also as “to render impotent”, “to deprive of virility (emasculate)”, or “to deprive of vitality, strength, or effectiveness”.
The word “castration” has a double meaning in Benjy’s case: it is both a physical attribute and a description of his situation. The fractured nature of his mental state leaves him unable to effectively communicate with virtually anyone. The only person he is able to develop any sort of meaningful relationship with is his sister Caddy, and this is only because Caddy is singularly compassionate towards him and actively reaches out to, defends, and takes care of him—something which is looked on with increasing distaste as Benjy gets older. Benjy is very alone and is unable to do anything about it. His castration is not only symbolic of the fact that his branch of the family tree must now also be a lonely one, but also that he is set apart from others and is not an effective member of society. In a society that puts a great deal of weight on the idea of white masculinity and superiority, Benjy is left at a distinct disadvantage.
The concept of white masculinity in the Southern society of the time already frowns upon the idea of men being fragile in any way, physical or mental, and the fact that mental support and understanding is something that Benjy clearly, desperately needs does not endear him to those around him. He is expected to be strong and self-sufficient, and not rely on others to protect or take care of him. Their Mother views Caddy’s attempts to help Benjy as her spoiling him, and insists that Caddy must stop trying to carry him. Despite Caddy’s insistence that Benjy would be better if given some help, the family still sees Benjy’s predicament as a behavioral issue as opposed to a mental health one. He is deprived of all the attributes that are considered by the society in which he is living to be important for someone of his gender to possess—and he is punished harshly for it. The few stereotypically masculine traits he does have are subsequently taken from him, and he is left even more of a social pariah than he was before.
When Caddy leaves, Benjy is put into a situation in which the one person who has always actively looked out and fought for him is gone. He is left even more open to the harsh judgment of those around him and is left fully subjected to the harsh Southern views of white masculinity and gender roles. Without the buffer of his beloved sister, he feels his castration even more keenly. Despite everything, Benjy is still painfully aware of certain aspects of his predicament; however, he is helpless to change his fate.

