Hamlet

So Hamlet is essentially this overgrown teenager whose mother has moved on from her recently dead husband with her husband's secretly murderous brother; his girlfriend's father is a nutcase spy-machine who is obsessed with status and appearances and will stop at nothing to expose Hamlet's faults to Hamlet's father-uncle; he's a sensitive, brooding, pensive boy-man who is constantly reminded of the importance of manliness by all the men surrounding him and is likewise reminded of his relative impotence--and all these factors play into the quiet man's transformation into a near-insane, callous murderer.
It's clear that Hamlet is a troubled man and his choice self-defense mechanism is high-grade narcissism. Narcissists see themselves as more valuable than other people; smarter, better looking, more desirable to be around--you name it and they're better at it. In their minds of course. Hamlet may not feign popularity, but he sure as heck believes he's intellectually and morally better than his counterparts.
We've seen Hamlet make a fool out of Polonius, Rosencrantz, Guildenstern, Claudius, Gertrude, Ophelia, even Laertes in the face of death. He treats people like literal shit. Part of him believes they are actually dirt on his shoes and part of him is so lonely that he somehow can't face people and so he isolates himself from them even further.
When Hamlet insults Polonius, it seems as if it's not malicious, just in self-gratifying jest. Hamlet doesn't like Polonius, but he's not spiteful. At this point, he still has Ophelia, and so he hasn't lost his humanity yet. The old man just proves to be a fun chew toy for Hamlet to toss around.
Rosencrantz and Guildenstern's betrayal hits home a bit more than Polonius', as they were Hamlet's childhood friends. Yet in the episode where the old friends match wits, Hamlet may make them look stupid, but still he is not malicious. Hamlet only becomes cruel when his one tie to the earth is cut.
When Ophelia betrays Hamlet in favor of her worthless father, Hamlet's heart breaks for what seems the final time. She is his last hope and she fails him. She is what rooted Hamlet to the earth--as we learn in the end, no matter what the interpretation throughout the play, Hamlet truly loved her in the end. He seemed to be able to handle Polonius, Rosencrantz and Guildenstern's betrayals---due to his inaction he might have been bearing his mother's and his uncle's betrayal.
But Ophelia's betrayal sends him over the edge. Sends him into a craze during the play within the play, sends him to his mother's room with near-murderous intentions, causes him to jump and stab Polonius who hides behind the curtain, pushes him to callously order Rosencrantz and Guildenstern's death. And let us not forget the way he treats dear Ophelia. He degrades her. He abuses her. He may physically harm her. He absolutely dehumanizes her and his actions might lead to her suicide.
Ophelia is the person who ruined Hamlet. Polonius may have done it indirectly, but it was Ophelia who sent him over the edge. Her role as Polonius' whore catalyzed the proceeding action in the play.