Why was Tom "in agony"?

Tom was in agony waiting for Huck. The two boys were heading to the graveyard.

Tom was in an agony. At last he was satisfied that time had ceased and eternity begun; he began to doze, in spite of himself; the clock chimed eleven, but he did not hear it.

At the end of half an hour they were wading through the tall grass of the graveyard.