I follow Marvel issues religiously and, indeed, I have never thought about this question above, raised by Wonder Man. He had just returned and faces the Avengers confronting them with all the bad they've done through their time as a team. I thought about it and, actually, it’s true. Heroes sometimes mess up. Following there is a list of some negative events or aspects caused by those who claim to be the heroes.
1. The Avengers created Ultron:
As we know, Ultron’s a robot practically indestructible created by Dr. Hank Pym as the Ant-Man, active member of the Avengers. Initially, Ultron was a project that ended up rebelling against his creator and developing his own intelligence. Ultron is one of the greatest villains in Marvel’s universe and the direct result of an experiment gone wrong. Ultron will be the next villain in the Avengers movie as we all know, and I believe that every Marvel fan hopes to see this dark side of the heroes’ failure and the consequences of their acts.
2. The Scarlet Witch
What happens when the heroes go crazy? Scarlet Witch is a mutant with very complex powers but we can summarize them as magic and reality manipulation. Many times she is accused of being crazy and that fact was proven in the M-Day events, when the Witch simply exterminated a big portion of the mutant population that were reduced from millions to a few hundreds.
Those issues were really so important that they have repercussions still today, and they happened almost nine years ago. Scarlet Witch was and still is a member of the Avengers.
3. The Sentry
The Sentry is some kind of superman in Marvel Universe. He has the power of a million burning suns. But what happens is that this superman is crazy. He was easily tricked by Norman Osborn to join him and his Dark Avengers. He has also an alter-ego called the Void that, well… turns him upside down. He ended up being killed by Thor after the events in The Siege.
4. The Hulk
Yes, in the Avengers movie he is quite controlled, but what will happen when the big green monster go nuts? (I think we are about to see it in the next Avengers: Age of Ultron). A fact is that, during his time in the comic books, Hulk was out of control 70% of the time. He is frequently mind controlled and hypnotized. For example, Tyrannus was once capable of dominating the beast and cause death and destruction in his name.
Yes, Venom was first seen on earth by Spiderman. He got his black suit during the events in Secret Wars. Later, he discovers that the black uniform was actually an alien symbiont. That symbiont joined later Eddie Brock and they, together, became an entity called Venom, one of the most adored Spiderman villains.
As if this wasn't enough, Venom is the “father” of another supervillain, one even crazier and more blood thirsty, the Carnage. So we can say that, indirectly, Spiderman is responsible for the birth of these two monsters.
6. Civil War
This was a major issue in Marvel history. Heroes fight against heroes over the question: Shall all the individuals with super powers reveal their identity to the government? This was, in fact, an extremely intense moment which put the team face to face. Tony Stark on the law’s side and Captain American leading the rebels. These events caused great destruction in the streets with all the physical confrontations including innocent deaths.
7. X-Men vs The Avengers
This was one other issue where the heroes forgot what surround them and focused on their differences, especially Scott Summers and his X-Men, who decide to stick with idea that the coming of the Phoenix, a force that only caused damage and destruction, would not cause damage and destruction this time. This caused more inevitable confrontations in the streets.
This is just a small list of the effects of the destruction caused by heroes. I believe the fans would actually enjoy see this dark side of the heroes of the screen, especially Civil War (It’s a shame that Avengers vs X-Men can’t be done… for now). Let’s wait and see how the unpleasantness between Captain America and Tony Stark is started and developed and what will the issue in Captain America: Civil War be all about.