The way I understood it was that he wanted to show Guts he had no power to stop him, even on a battlefield where he is at his best, like that shyt was personal.
- He knew those two had gotten close because he saw how they acted with one another I believe when he was crippled and possibly at that dance too. And its not that he wanted Casca, its because he could always rely on her being with him in case he wanted her like a side piece plus she saw him like he could do no wrong. But then Guts.. just came in and took her and she started to doubt him. If he had his organization well off and running (Guts staying) and could continue his plans I don't think he would have mined. But losing her and guts + his band + his prestige and his body was too much for him.
- Another thing, I feel like to Griff the band/organization being dismantled was Gut's fault. Because he knew without him that it was all gonna fall apart. It had something to do with a prophecy or something he was told, so he was like a good luck charm as long as he was with him in addition to being his best fighter. This dude is twisted though and possibly a little..
.. he saw Guts as a possession, like everybody else on a board that he could sacrifice when needed and they should be grateful for it because he made them who they are and..
he's destined to be somebody important
So when he was doing what he was doing it was very personal
like on some payback/revenge. If only the author would wrap this shyt up though man, this evil b*stard should have been gotten that fukking work by now