What is the 'subtlety' of Tom's predicament as Scout sees it?

The subtlety of Tom's predicament was that he had no choice..... one option was no better than the other.

From the text:

Until my father explained it to me later, I did not understand the subtlety of Tom’s predicament: he would not have dared strike a white woman under any circumstances and expect to live long, so he took the first opportunity to run—a sure sign of guilt.