This scene couldn't have happened; Arthur's Mordred is male.
Also a lot more psycho than our Mordred is capable of being.
Alright, so a bit of clarification to that:
Classic Mordred's main struggle is that they want to be acknowledged as Artoria's heir. That's the core of it. Proto Mordred is seemingly more volatile because his case is emotional neglect; with Arthur constantly out in the open fighting the Saxons and the Picts, he's never home for his son. Proto Mordred just wants his father's love; he's an emotionally starved child who just wants his dad to, well, be a dad to him.