The Duke confides to Friar Thomas that he has been lax in upholding the laws for the past nineteen years: "Which for this nineteen years we have let slip." He says that if, having been lax, he were to strictly uphold the "biting laws," it would be like sudden tyranny on the people. Since he blames himself and his lax attention to his duty, he says the people's lawlessness is his fault.
I do fear, too dreadful:
Sith 'twas my fault to give the people scope,
'Twould be my tyranny to strike and gall them
Angelo is a lord and Duke Vincentio's second steward. Angelo has a reputation for being "precise" in governing his emotions, temptations, and "appetites." Consequently, the Duke has decided to invest Angelo with the power to rule and administer the laws while the Duke is supposedly called away on urgent business. The Duke confides in Friar Thomas that his intention is for Angelo to bring back adherence to the letter of the laws so that the people don't associate the new strictness with himself, the Duke.
I have on Angelo imposed the office;
Who may, in the ambush of my name, strike home,
And yet my nature never in the fight
To do in slander.
We can say that--based upon Angelo's private thoughts expressed in a soliloquy after Isabella leaves him--Angelo's morality changes with the first sexual temptation that comes his way.
Shall we desire to raze the sanctuary
And pitch our evils there? O, fie, fie, fie!
What dost thou, or what art thou, Angelo?
Angelo arrests and sentences Claudio to death for impregnating his betrothed before their marriage. Isabella, Claudio's sister, who has just joined the convent as a nun, goes to Angelo to plead and beg for Claudio's life. After much importuning, Angelo begins to a see wicked light and chooses to bargain her physical love for her brother's life.
... Even till now,
When men were fond, I smiled and wonder'd how.
Thus we may say that after Angelo is deputized to rule in place of the Duke and after he has met and listened to the pleadings of Isabella, Angelo undergoes a radical change in morality and begins to covet the temptation that he is about to put Claudio to death for.