I still feel it's a lot deeper than that, not just ego, and not just control. Did Griffith love to control Guts? Sure. He loves control and being in control.
But he lost someone because their dream didn't align with his anymore. When Guts listened at the fountain and found out what Griffith saw in others, he felt the need to rise above being a simple tool, a weapon, and be someone. This unfortunately took Griffith's favorite "toy" away - but he realized (when it was too late, after Guts won his freedom) that there was far more to it. Even during the eclipse, Griffith was absolutely torn on what to do - and it was how vulnerable Griffith felt, remembering how much it hurt when Guts left - not just the loss of his favorite toy but a sense of togetherness that he felt with Guts that he didn't feel with anyone else, period, that hurt him so much and swayed him, in the end, to turn into Femto.
It's complicated, emotionally - and he loved Guts very much. Love makes you vulnerable, and love is what pushed Griffith to sacrifice what he loved. The pain of having to sacrifice someone you genuinely care about in order to get what you want, and Griffith dealing with the realization that if he stops now, everyones' sacrifice was in vain. It's a tough choice. Obviously he should have decided not to, but that would have been the end of everyone, anyway.
