This is a very theological question.
I think the answer would be that God didn't decide to be holy, but that he iss holy. In the same way that he doesn't 'decide' to be righteous, or love, or truth, etc. This is simply how he is. And this is how he presents himself in the Bible.
I never heard of any verse which said that God 'decided' to be holy. Only verses that said that he is.
Of course, you are spot on that hell is the consequence of his holiness. But it is the because of what he is, not what he decided. If it really was a choice on his part to send us there, why would he ever have to become a man, and suffer and be humiliated and ultimately die. It makes no sense at all, since all he could have done was change his mind instead of having the crap beat out of himself ... (Well, his son. But ya know. Trinity and all)