In Shakespeare's Hamlet, Polonius, having to insert his opinion into everybody else's business, can't let Laertes' final words to Ophelia before he leaves in Act 1.3 go without comment:
Farewell, Ophelia, and remember well
What I have said to you.
Polonius asks Ophelia what it is that Laertes said to her, then lectures her on how her actions reflect on him and her honor. Productions of the play usually depict Polonius as being very forceful when he orders:
What is between you [Ophelia and Hamlet]? Give me up the truth.
He then dismisses what he sees as Hamlet's affection--"Affection? Pooh!"--and tells Ophelia, basically, that Hamlet is using her for sex and that she shouldn't believe him when he says he loves her. He, then, orders her to have nothing to do with Hamlet.
He later apologizes to Ophelia, admitting that Hamlet may actually be in love with her. But he ruins this realization by then ordering her to spy on Hamlet for him and Claudius.
In short, Polonius bullies his daughter, doesn't listen to her, is concerned only with her absolute obedience to him and not her feelings, and worries that she will reflect badly on him.
Trapped in a patriarchal society, Ophelia is reduced to an extension of Polonius. When Hamlet reacts to her spying on him with cruelty, and when her father is later killed, Ophelia is not prepared to handle herself in such situations. She becomes unbalanced and ultimately commits suicide.
No doubt Polonius loves his daughter; however, he is clearly more concerned about his own political position than he is about Ophelia's welfare. Examine the evidence:
When Polonius discovers Hamlet and Ophelia have become close, he makes her promise to sever the relationship. He expresses his disbelief that Hamlet wants anything from Ophelia other than sex--an insulting thing to say to any woman, let alone a daughter. Despite his ungracious comment she is honest and obedient, while he is only worried about the ramifications of this kind of affair on his career.
When Polonius realizes Hamlet may actually have true feelings for Ophelia, he makes a plan to "catch" Hamlet in his madness, Polonius makes this proposal:
"You know sometimes he walks for hours together/Here in the lobby..... At such a time I'll loose my daughter to him."
This animalistic language makes her nothing more than "bait" to lure and capture the prize. What happens in that fateful meeting with Hamlet is the direct cause of her madness and eventual death. It's intriguing to think about how Hamlet tried to spare her but her father was too self-absorbed to see beyond himself and his own ambition.
Ophelia commits a passive suicide and must certainly take some blame for her own action; Hamlet, too, bears some responsibility for Ophelia's demise. However, Polonius' parental influence certainly helps put her in the grave.