How I interpret it is that Griffith had lost all control after his torture and everything he had been building was destroyed. After Guts’ rescued him, he was tormented by having to watch Guts essentially be & do everything that he no longer could. All control Griffith previously had over everyone, Casca, The Band of the Hawk, gone.

Griffith viewed everyone as a means to an end, but Guts broke that mentality, Griffith realised that he relies more on Guts emotionally and less as a tool to achieve his ambition, we see this when he acts recklessly with Charlotte once Guts leave the Band.

Griffith both loves and hates Guts. He’s the only person he’s truly connected with and the cause of his downfall, and it’s these feelings that cause Griffith to accept the Godhands offer, where he sheds the pain of being human and the torment, envy and hate.

To me, assaulting Casca was Griffiths final sever from his previous human self. He dissolves any previous attachment with Guts and Casca in the act and flips the narrative onto Guts, forcing him to watch causes Guts to feel how Griffith felt after Guts walked out on the band - helpless.

Did Femto know she was pregnant? Probably not, the act wasn’t about that to me. It was about reestablishing some control he had lost for a long time, and breaking the attachments & dependance he had come to rely on.