I suppose that this could be debated, but I would argue that the worst possible crime that someone could commit in these times would have been the crime of regicide -- the killing of one's own king.
Back in these days, the world was seen in very hierarchical terms. People all had their appointed (by God) places in the world and they were not supposed to try to get out of those places. On the top of the heap was the king.
If you killed the king, you were upsetting the whole order of the world. Not only that, you were upsetting a world order that had been decreed by God. So that would be a very heinous crime -- much worse than something like murdering a normal person.
We wouldn't think this way now because we don't believe in such a hierarchical world, but back in these days, things were much different.