What are some of the most shocking and/or twisted moments in comic history? Some of the stuff that really sticks out for you?
The Death of Superman

Looking back at it now it just seems cliche. But I remember when it happened that the public really reacted. It wasn't just another comic event. The real comic fans knew Superman wouldn't stay dead, but the media had a field day with it and the general public reacted like we were actually losing Superman. It was pretty cool that comics made that big of an impact at one time.
Kyle Rayner's Girlfriend's Body is Stuffed in His Fridge by Major Force:

Poor Alex DeWitt. This was out of left-field as the villain Kyle was fighting was nothing special, nor was the storyline, well at least until this happened. This event scarred Kyle Rayner for years, and was quite inventive for the dull-witted villain.
The Joker Beats Robin to Death:

He's running around with the living thanks to Superboy-Prime's "reality punch" 8-| But before time got messed with, Jason Todd was without a doubt six-feet under. Having been beaten to a bloody pulp by The Joker, with a crowbar. Brought Joker back to being a scary villain after years of being a caricature of himself.
Speedy is a Junkie:

The cover says it all. And with this particular issue, DC Comics ditches the Comics Code Authority.
Batman is Beaten:

I'm a child of the 90s, and this event I remember right alongside The Death of Superman. Batman was invincible in many people's minds. Sure he lost a fight here or there, but he always found a way to win. This time he didn't, and his back was broken. It seemed Bane had won. Maybe this was just shocking to a 10-year old, but I couldn't believe it.
Hal Jordan Snaps and Destroys the Green Lantern Corps, VERY Suddenly:

It's like it happened overnight. One day Hal Jordan is patrolling space, saving innocent space men and the Earth alike. Then BOOM! He's on a hot tear to the Planet Oa ready to open a can of whoop-ass on The Guardians of the Universe and any other Green Lanterns standing in his way. In the end Sinestro, The Guardians of the Universe, and The Green Lantern Corps were gone and dead. Hal Jordan became the cosmic-villain Parallax, and Kyle Rayner (a completely new character) emerged as the new and now one & only Green Lantern in the DC Universe. It was like a kick to the gut for Hal Jordan fans. But at the same time an exciting addition to the Green Lantern mythos.
Ultimate Red Skull is Revealed to be Ultimate Captain America's Son!:

And this is after he already ripped off his the skin of his own face after killing 200 men. That's a dysfunctional family!
Peter Parker is the Clone!

Okay, so that wasn't the truth when it was revealed, but for those of us reading the Spider-Man books we thought Ben Reilly (Scarlet Spider) was the real Spider-Man for like the next 3 years. This caused a such a controversy with fans that Marvel actually gave in to fan's demands and made Peter Parker the true Spider-Man again.
Magneto Rips the Adamantium from Wolverine's Body AND Professor X Mindwipes Magneto in the Same Issue!

This issue was AWESOME back in the day. Wolverine lands a serious wound on Magneto causing him to react in a deadly fashion. Not only did Wolverine's Adamantium get ripped from his body nearly killing him and working his Healing Factor to it's limits. But as a result, Professor Xavier did what he vowed he would never use his powers to do to anyone: he mindwiped Magneto leaving him a drooling vegetable. Unknowingly creating the future-menace Onslaught in the process. (Whew!) Plus you got a cool 90s-style hologram on the cover!
Captain America is Assassinated

Another one that's been undone by now. But it was cool when it happened. And it actually was done for the sake of story and not just the shock for once. Civil War was the best crossover event to happen in comic books for years. And just when you thought it was over, Captain America is assassinated with nothing more than a single bullet. And this was right after Captain America became the bigger man and put down his shield in the name of peace. It was very well done.