Part of it is probably also an attempt to reestablish his once dominant position over Casca, considering that he had already (pathetically) attempted to force himself on her somewhat in the scene where he sort of throws himself onto her (though, that could just be solely to keep Guts around by making Casca pity him and also stay). In general leading up to his sacrifice you can get an idea of his fucked mental state by examining his reaction(s) to Guts and Casca’s relationship and — more importantly — progress without him, etc.