This realisation by Scout is true because, although all the evidence suggested that Mayella was actually attacked by her father rather than by Tom, no jury in Maycomb would ever have convicted Bob Ewell over Tom Robinson, simply because Ewell was white and Tom Robinson was black. It was easy enough for Ewell to pretend that Tom had assaulted Mayella, as most people in a prejudice-ridden community like Maycomb would readily believe the worst of a black man anyway, and he evidently also persuaded, or more likely forced, Mayella to lie as well. In a town like Maycomb, all power rests with the whites. Tom simply stood no chance at all of ever having his word believed over Bob Ewell's. Even if there were individuals ready and willing to help him, when it came to actually delivering a verdict, the (all-white) jury would automatically pronounce in favour of the white man - even one as generally despised, and as despicable, as Bob Ewell. So, from the moment that Mayella was attacked, Tom was already effectively convicted, and sentenced to hang.