The male Malabar spider, even after losing its genitals, remains a tough fighter that is still able and willing to take down opponents. In fact, genital loss makes it more aggressive. I’d be mad too.
Simona Kralj-Fiser from the Slovenian Academy of Sciences and Arts in Ljubljana thinks there may be an explanation for this. The eunuch male has everything to gain and little to lose. To understand this you have to realize that male Malabar spiders leave their testicles behind after inseminating the female as a way to block the female copulatory opening and scientists have found that this works 75% of the time. However they can be removed by a rival male, who then gets to impregnate the female.
No wonder they fight so aggressively. They are protecting their investment, paid for with their own genitals.