The superhero has a tough job. Like celebrities, they're every action is under the scrutiny of the public. It doesn't matter how many good deeds are done or how many lives are saved. If a hero slips up and does something bad, their reputation is tarnished. They can no longer be trusted. There would always be the possibility they would mess up or do something dangerous again.
Fortunately, as with celebrities, the public has a short term memory. Often heroes fall from grace but manage to overcome their downfalls. This doesn't happen with everyone but it does occur often. Some heroes are just lucky when it comes to forgiveness.
A common reason a hero might temporarily turn evil is due to some sort of possession. As strong as the heroes are, even the most powerful telepaths can succumb to being possessed. Any horrific deeds the hero does while possessed would be unfortunate but you can't really blame someone that was not in control of their actions. The question is, how do you prove a hero was actually possessed and not just in a really really bad mood?
== TEASER ==One of the recent casualties of being possessed was Daredevil. He appeared to really go off his rocker. During Shadowland, he took it upon himself to claim Hell's Kitchen as his own. He had control over an army of ninja assassins. He murdered Bullseye out in the open. Bullseye may have deserved his fate but killing isn't what heroes do. Daredevil went up against his friends and collegues. After it was revealed he was possessed, the problem was solved. Daredevil was cleansed and took off for a short period in order to find himself.
Now Daredevil has returned to New York City as if nothing had happened. The public knows his secret identity even though as attorney Matt Murdock, he continues to deny it. He probably feels bad for what he did but he returned with an almost carefree attitude. Captain America sought him as since he's basically America's supercop now. After trading some light blows and a super-brief conversation, Captain America decided to let Daredevil go for now. All is okay. They'll "talk" about it later which really means it's never going to be brought up again. Cap may understand that Daredevil was possessed but would the public comprehend and accept the notion?
Being an attorney, Daredevil is one to know the full extent of the law. In order for him to be proven innocent of any and all actions he committed during Shadowland, he would need to be proven innocent without a doubt. How can you really prove he was possessed? There isn't a mark or anything left to show it happened. A telepath could be called in but would a jury be willing to accept the word of one, even if they were a court-appointed telepath? Since superheroes are often friends and stick together, how would the jury and public know whether or not the telepath's assessment was completely truthful? Superheroes could have secret friendships that the general public is unaware of.
Professor X was also possessed when he became Onslaught. As Onslaught he was responsible for mass destruction and also killed the Avengers and Fantastic Four. The fallen heroes were actually alive in the pocket universe created by Franklin Richards but they were still gone for a year. Professor X openly announced he was a mutant and in charge of the X-Men (this time while possessed by Cassandra Nova). Is Professor X easily forgiven because the public didn't know he had such close ties to Onslaught?
Possession by an evil spirit or even a powerful telepath isn't something that can visibly be proven. While the person that is possessed becomes a victim, their actions are too easily overlooked. It's too easy for them to get off the hook. They shouldn't have to be dragged through lengthy court trials but what if an angry hero decides to get angry and cause havoc.
Look at what Wonder Man did to the New Avengers and their mansion. He felt they need to stop operating as a team because they've allowed too many bad things to happen. When ignored, his decision was to form a team and attack the Avengers. Destroying their home and defeating the team isn't something that can be easily overlooked. Is he responsible for the costs of rebuilding the mansion? Does he have a right to make the decision about the Avengers or will it be revealed later he's not simply angry but got possessed by someone or something?
What about Angel? Even with Psylocke trying to telepathically keep the dark influence of Apocalypse at bay, he's still killed. Does that mean Warren is responsible?
There really isn't a way to safeguard against evil possessions. The heroes could try to get someone like Doctor Strange to come up with an anti-possessing spell that protects them but we know that's not really possible or feasible.
Being possessed and forced to become evil is completely unfortunate but it is a risk in being a superhero. There just needs to be some way to prove a rouge hero was actually possessed and not just in the mood to be a complete jerk.