But does he have to do it himself? Could he not order their deaths if that is the case?
I mean he woke up on the island with the two of them looking at him. He may have anticipated the situation, but it's not like he came of his own volition. It's the boy who went there and Griffith had no real choice in the matter. So he took action. In another context, yes maybe he could have ordered someone to do it and maybe that would have been a workaround, but that's very speculative to even bring up. Do note though that he may send apostles after Guts in the future.