Good people via fear

"It is hard to believe that even a good man would stop himself from stealing and doing all kinds of other bad things, if he knew he would never get caught."

Fear does keep people in check, but that makes society sound scary. It's not so much fear as the norm. It is the norm to obey the laws and do everything you can to stay clean, which puts doing bad things out of your mind. It's just not relevant enough to think about doing.
Add an invisibility ring to this equation and you would, of course, start small. Maybe steal your Tim's coffee if you didn't have the change and were in a hurry. Minor inconveniences would be resolved. And then one day, if you saw someone doing something terrible to the one you love, you may just kill them. But it would take a long time, and, assuming you didn't share the knowledge of the ring with people, "Hey, honey! You won't believe this!" the alienation would be a slow process. But then again, people are more inclined to take personal risks for those they love. People with families can't truly alienate themselves from society.

One, single, lonely man (or woman) with no money, family, and a nasty case of psoriasis would be your likely candidate to turn lethal. I like the "what if" of the origin of evil, because there is no sudden change, it's a slow process that brings circumstances to play that are normally just impossible.