These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Complete Monster: Red Mist/Motherfucker. While the movie adaptation made him more sympathetic, he was just as much of a bastard as his father in the comics. For one, he expressed sadistic glee when he thought he killed Hit Girl. But issue 4 of the sequel had him cross a bigger Moral Event Horizon: he killed a group of children after they told him where Kate lived, had his goons kill a neighborhood of people, killed Kate's father, and then raped her with his other cohorts. Even his own lackeys thought they went too far.
On the topic of Red Mist, in Kick-Ass Volume 2, in the space of an afternoon, Red Mist rapes Katie, kills her parents, several children & police officers. After it's all said & done, he managed to get Vic Gigante telling him that the police will no longer turn a blind eye to his actions and the various mafia families are all gunning for him.
Foe Yay: Red Mist being... Red Mist: "...and if I'm absolutely honest, I've wanted to see you in pain since the first night we met. I've even jerked off about it. Does that sound weird?" Yes, Red Mist. Yes it does.
The fact that he started with a group of children makes it even worse after the Newtown shootings.
Jerkass Woobie: Katie in Volume 2. Good lord, did she not deserve that to happen to her.
Memetic Mutation: The fruit of David's efforts to rescue people? People cosplaying as superheroes and taking pictures of themselves, which now goes past the hardcore geeks and to everyday people. Plus, a whole bunch of them actually start training to be superheroes like Kick-Ass, Big-Daddy and Hit-Girl.
Also, "Tunk," the new swear word that Dave's friends made up (basically the male equivalent of Cunt) catches on unbelievably fast.
Red Mist and his father manage to cross it at the same time, after setting up Kick-Ass and the others. First, they beat the shit out of them. When Hit Girl tries to fight back, Johnny G orders his men to shoot her in the back, which they gladly do, sending her flying out a window. Red Mist then raves about how awesome that was, demonstrating that he's either a total sociopath or at least isn't very good at separating comic books from reality. In either case, that's the moment when both of them stop being mere villains and cross the line into Complete Monster territory. Interestingly, the film turned this into a humanising Even Evil Has Standards moment for Red Mist by having him instead be suitably horrified.
Also the revelation that Big Daddy isn't an ex-cop and dragged his daughter into the lifestyle after running away from her and lying to her about the fate of her mother. This is one point where a decent amount of people seem to prefer the film version of the character over the comic due to the absence of this.
In Kick-Ass 2 they basically pull out all the stops to make the villains seem unstomachably cruel, clearly in an attempt to eradicate any surrogate thrill the reader might get from their exploits. Don't-give-a-shit wanton violence is such a primal power-fantasy, they really wanted it to seem as disgusting as it would in real life. Most common criticism is that they went too far in this, parts of the book are just really unpleasant to read.
The Scrappy: Big Daddy for basically turning his daughter into a killer just because he was bored with his life.
Ship Tease: For better or for worse, a lot of the scenes between Kickass and Hitgirl are framed from an odd angle where it looks like one is kissing the other (the dialogue bubbles dispel the notion, but the imagery is there), in addition to the two being about as close as anyone in the comic can be. The very large age difference may put it into No Yay territory, but Hitgirl being both more mature and more experienced may not, so it's a YMMV/perception thing.
Stoic Woobie: Hit Girl. However, not so much after her father died.
After utterly destroying John G and the rest of his goons, Hit Girl finally breaks down.
Hit Girl: Would you give me a hug? My daddy just died.
Wangst: Dave's narration can be accused of this. But everyone else have their moments too.
What an Idiot: Pointed out by Kick-Ass in Volume 2, after he & Hit-Girl find out that the Mother Fucker's plan is to burn New York to the ground - Chris is asthmatic, so what's he going to do when he blows up all of the pharmacys & hospitals, and he needs his medication?
Crosses the Line Twice: A lot more than twice actually. Single scene example: Dave gets stabbed in the gut. The audience winces. He then staggers out into the street and gets flipped by a car. The audience starts laughing.
Moral Event Horizon: D'Amico is most firmly established as a complete psycho-ass when he murders both some guy unfortunate enough to be wearing a Kick-Ass costume and an innocent witness to said murder.
Of course, he did believe it was the real Kick-Ass and that he'd been killing D'Amico's men. He really crosses the line when he finds out Kick-Ass had nothing to do with it but still insisted on him being beaten and executed on a live webcast. The sheer look of sadistic pleasure on his face and the disgusted look Chris gives him when they're both watching it said volumes about what a Complete Monster he was.
Wangst: Same reason as the comics, as mentioned above.