I don't want to debate, because I totally agree that Finn was perfectly respectable during the break-up except for the heartless comment. Rather, I'd like to defend Quinn, because the Fuinneral kind of connected a few things that have been building up over the past few weeks.
Quinn used to be "ugly," and when she got the opportunity, she was weak and changed herself. It juxtaposes her from people like Rachel. And to anyone that says that storyline was about her and Lauren, it wasn't. It was setting up for the separation between the weak (Quinn) and the strong (Rachel), which is why I Feel Pretty / Unpretty went to Rachel and Quinn rather than Lauren and Quinn.
We know why Quinn has been such a bitch - she views it as something she must do to have status. The personal bullying to Rachel is quite obviously because she sees herself in her, which was evident through both of them looking in the mirrors during IFP/UP.
Prom Queen is the pinnacle of popularity. In Quinn's mind, it's everything she has ever worked for. She came up from Lucy Caboosey to get the life she'd always wanted, and in a literary sense, prom queen represents that life. Which was why, narratively, her secret was introduced in the height of her campaign for prom queen.
Everything about Quinn's search for power was wrecked by one bad decision, though. Having sex with Puck. If she had never gotten pregnant, I'm convinced very little of this would have happened to her. She had sex with Puck, which threw away everything she had worked for. In Season 2, she got it all back, becoming popular again. Her nice streak that came from her character development in Season 1 is what pushed her to Sam.
Now, people say Sam is Finn 2.0, but he's not. Finn represents confusion, Sam represents the anti-Finn -- picking out the important things. It's what he has to do as a dyslexic, and it's why he's been written to be homeless. It's also why he's currently paired with Mercedes, because she's the only girl to realize that you have to look at the important things. But that's not what this is about.
Sam might not be as popular, but Quinn realized that she didn't need the double-whammy to truly get what she wanted. She had Cheerios, so that was enough, and she could afford to be happy. Her speech about glee in Duets and her singing Time of My Life is narrative evidence. Also, the fact that she won the Duets competition narratively signaled that for the first time since Preggers, Quinn was on top.
Finn convinces her, however, to quit Cheerios. Probably influenced by some lingering feelings because, as we know, she's not completely heartless (Funeral). Those feelings were sort of foreshadowed in Britney/Brittany. Without Cheerios, though, she's nothing, so with Finn in the position of power he's in, she goes after him, tossing Sam to the side. In the end, she decides that's not right, and goes back to Sam, having her own 'come back'. Unfortunately, it gets tossed in her face.
So she hits rock bottom, runs back to Finn, who willingly accepts her because she represents everything that went wrong in his life. If he can have her, and make it work with her, he's just fixed everything that came toppling down. Unfortunately, he's still in love with Rachel.
She's so content to stay in Lima because Finn is now the only thing she has left. She thought Sam would bring her happiness, but he shot her down, and now prom queen is the key to achieving that victory that she's been working toward. In Comeback, she thought that just being happy with Sam would be enough, but now, prom queen is the goal. Prom Queen will represent her defeating her former self.
But wait! Kurt wins prom queen. Her victory is turned into a joke, and she's comes crashing down. She slaps Rachel because she believes that Finn leaving over her is the reason why she lost; she believes this ghost of her past kept her from winning what she's worked so hard for, and in a way, she's slapping down her past. But Rachel doesn't leave, because Quinn's past will, symbolically, always be with her. And in a way, that's what leads to Quinn's desperation during the Fuinneral tonight. Her past still hangs there, and as much as she knows she has to accept it, the last time she did that, Sam broke up with her, so she stays with Finn. Then Finn breaks up with her, and she jumps to Rachel, who, I'm going to say again, represents the past. Finn is her tool to get that victory now that she can't go back to the Cheerios, and he's choosing her past over her, which in a way, nullifies all the work she's done up until this point. It would be as if everything was for nothing.
She's an emotional trainwreck, and her ex-boyfriend is sitting here calling her heartless. Sure, everything has been a plan, but she's human real feelings are laced in here. She's parallel to Sue ("you remind me of a young Sue Sylvester, Q") in that she's always striving for victory, but there's always some sort of emotion laced into there. With Quinn, it's usually her affection for her boyfriend at the time. With Sue, it's Jean. Finn accusing her of being heartless is parallel of Will accusing Sue of being heartless for cutting Becky at the episode's beginning. Neither of them, then, know how to handle the grief, but the problem with Quinn is that she doesn't look to anyone for help. For Sue, there's Kurt and Finn, but for Quinn, no one knows her pain - well, except for Rachel, but her pride obviously won't allow her to go there. So she's got a secret plan, and we'll probably have more to add to this next week.
Too long, don't read: Quinn is a complex, sad character, and everything has sort of just been coming to a head for her these back episodes. Poor girl, if only you hadn't had sex with Puck.