How is Angelo corrupted in Measure for Measure?

In Measure for Measure, Angelo imprisons young Claudio, convicted for seducing Juliet, and sentences him to be beheaded. Claudio sends word to his sister, Imogen, who is that day entering the convent, to please go to Angelo and plead for his life. Imogen pleads fervently but Angelo is tempted by her beauty and tells her he will spare her brother if she will lose her virtue to him.