Well it's all dependent on circumstances and situations. Many of these individuals were only turned evil because events in their lives cornered them so much that after they couldn't handle the pressure, they just broke and released all their negativity in one blow.
A big example? Higurashi no Naku Koro ni's characters. Every single one of the main cast is a legitimately lovable, awesome character that I would love to be friends with and came to care about so much as the story went on. The catch? A good portion of the anime pretty much revolves around the stories of how their situations break them and cause them to become desperate, psychopathic killers who are utterly shattered by the circumstances. None of the events are truly their faults and the more you watch it, the more you realize that they are really just like any other average person in a regular situation but when things really start to go heartbreakingly wrong? Add on some of the plot elements of Higurashi itself and you've got a recipe for disaster.
In general, very few people are really born with the desire to be dark, evil spirits and all. Obviously some examples (Dio from Jojo's Bizarre Adventure for one) are cases where they're pretty much evil from the start and you can tell. But more likely than not the villains were probably initially kind, caring people who were shattered so much by the tragedies of the world that it continued to taint them and dominated the way they viewed life, taking a turn for the worst. And it's a really sad thing.
As for whether they can go back to good? Well again, it's really all circumstantial. Looking at Higurashi, in a way they can't because in the arc where the characters break, many of the cast usually ends up dead and that's that. But at the same time, the arcs of Higurashi (which seem like totally separate stories at first) all connect and as the characters start to recall their tragic mistakes from the previous arcs they realize just how important their friendship is and this eventually results in probably the most badass showing of unity in the entire anime so yes, I do believe that with the proper circumstances it is very possible. Though Higurashi is a weird story and with the way the arcs go in a way you could say that in that case they were never evil because they never went through the circumstances in that arc that would push them to darkness.
