Polonius, while a comical stock character, is also a sycophant with a penchant for spying. He also worries that the actions of his children will possibly put him in a bad light with the king. So, in order to gain favor, he goes to King Claudius and Queen Gertrude, knowing that they are concerned about Hamlet. He reports that Hamlet has written a letter to his daughter Ophelia that indicates that he is "mad." Further, he affirms that he has come to inform them immediately, and that he has given his daughter "prescripts," telling her that she must lock herself away from Hamlet and repel any of his advances, and she has dutifully obeyed.
'Lord Hamlet is a prince out of thy star.
This must not be'. And then I prescripts gave her,
That she should lock herself from his resort,
Admit no messengers, receive no tokens.
Which done, she took the fruits of my advice;
And he repelled, a short tale to make,
Fell into a sadness, then into....madness.(2.2.139-148)
Having realized that Polonius has exploited his daughter and is listening behind an arras in order to learn something which may incriminate him, Hamlet becomes cruel to Ophelia, saying, "I did love you once," rejecting her now. Later, before The Murder of Gonzo begins, he speaks lewdly to Ophelia and tells her to go to a nunnery so that she will stay away from him. However, the sensitive Ophelia is crushed by Hamlet's cruel words and goes mad herself, eventually committing suicide. Clearly, her father's exercise of his power over his daughter for his own political gains contributes to Ophelia's suicide.