The problem wasn't that Will sought out revenge. The problem Lex had was the slaughter of so many innocents along the way. Honestly, Lex was certain that if he faced something similar, his revenge would have been even worse. Just killing his enemies was not enough - he would make them live in regret and suffering.
But… even in his grief, he would not target innocents. Destroying an empire could have been done in other ways. Even if he found an empire of his own, and slowly took over his enemy's empire, it would have caused fewer deaths - not that Lex was condoning such a thing.
Although, from his own point of view, the old man was simply seeking revenge, he had become the villain in so many other people's lives.
The old man's body trembled. He had expected condemnation or hatred, maybe even accusation. What he had not expected was to be faced with words filled with such profound pity. He felt them in his heart, in his soul.
