The eponymous Miss Julie is nothing short of unstable, with sadomasochism being her key disorder. We know this because her mother, the Countess, was basically the same way, going as far as burning down the family's estate and then asking her husband to borrow money from her lover to re-build it. Julie was just like her mother; she hated men, and hated herself no less.
The phrase "I'd love to see the whole of your sex swimming in a sea of blood just like that" is a reaffirmation of her hatred toward men. This takes place right after Jean takes Julie's bird, Serena, and decapitates it. The purpose of this was to be able to elope leaving nothing behind, much less leaving her beloved bird with strangers. Julie, at this point in the play, is more unstable than usual. She first agrees to the idea of killing the canary but when Jean actually does it, she (Julie) abruptly changes her entire mindset of the whole situation with Jean (that is, their idyllic elopement) and waxes hysterical, yelling a very long rant at him.
The specific phrase about "the whole of your sex swimming in blood" means that she wants ALL men to drown in blood and disappear, just as easily as Jean kills the bird. She cannot believe that Jean went on with the plan of killing the canary. Moreover, now Julie sees Jean as yet another male enemy, this one having the gall of taking away her bird companion.
After this, Julie's rant goes on to directly attack Jean, his lowly social status, and even his future generations
I'd like to see your whole sex swimming in a sea of blood, like my little bird...I think I could drink from your skull! I'd like to bathe my feet in your open chest and eat your heart roasted whole!---You think I'm weak. You think I love you because my womb craved your seed. You think I want to carry your spawn under my heart and nourish it with my blood--bear your child and take your name!
And the rant does not even stop there. She then emasculates him by reminding him that he has no family name, that she would be bettering his but not hers, that he will cause her father to have a heart attack and die, and that her good name will essentially die with her. That would be Miss Julie at her best.