Though confident in his own innocence, Randall was disheartened to find that this venal character would be serving as judge over his trial. Based on the sentence, the word venal most likely means:

The word venal is an adjective which means that something or someone can be bribed or corrupted. In this case, the word venal means that Randall's immoral attitude or character will serve as a proof against him in trial. That is to say, his usual wrongdoing will be the one which ultimately will condemn him.

Easily corrupted - Apex answer