Does it go the other way? Are there people who spend their life as good (a more or less model citizen) becoming bad? Perhaps not, because it seems more natural for humans to want to contribute, so even criminals may have a voice in them urging them to do better. But humans do have thoughts of envy, lust, self-righteousness and unfairness that can whittle away the self control and cause them to cross over and, let's say, murder someone in the first degree.
I'd propose that a complete set of research would learn that there are some personality traits that are malleable and there are some that are not. Moreover, those two sets would not be the same for everyone, and there is likely a great deal of diversity.
Our minds bias towards telling ourselves that we're okay, most of the time, and we rationalize constantly in order to get through the day, imposing our storytelling techniques on capricious events.
That means that we behave "good" just to the extent that we're aware that we might feel guilty and not much farther - and in that sense, we can train ourselves into greater moral fortitude with deliberate practice in empathy, exposure to diverse views, and development of a personal ethical philosophy. I don't see "personality" as crucial to that - it can differ - and changes in environment and status have a huge impact on what we perceive as acceptable, as well as medically diagnosed conditions that literally disable our ethical senses. Across a population, what is probably most important is to have a little bit of wisdom distributed among everyone so that we aren't burdening each other constantly. "Crossing the line" into violence happens after a huge, huge number of other failures have already taken place.
Systemically, we aren't even to the point where we might discuss this as a problem, though. We don't yet see wisdom as something everyone needs in the same way that we think that everyone should be fed, housed, and clothed: If you have those things and you're unhappy, the systemic recourse is to engage in consumer activities marketed around your perceptions of unhappiness.
>Are there people who spend their life as good (a more or less model citizen) becoming bad?
Of course. You think all of the Nazi officials were rotten from their childhood, teenage years, early career, etc?
Some were -- most not. They were "model citizens" as you say. It was the circumstances and belief in Fuhrer etc. that led them to become bad, but after some threshold we can say they were bad.
There are some people who simply become bitter, angry at everything and that gives them difficulty making new friends... I can't recover my faith in women since being dumped in my 20ies in bad conditions - it gave me an entirely new perspective on how society treats women (and I say "women" as the category fixed by the laws which gives those women rights because of their gender). And even if I keep trying to do the things of the good guy (thousands of donations, work for gender equality, church, volunteering...), I now rather identify with the bad person in the movies or in the political discourse. I've gotten used to defending a point-of-view that is slightly less inclusive because it relies on slightly more responsibility from citizen, and immediately get conflated with extreme-right opinions, so why bother?
The main argument of the article, supported by quite a bit of evidence, is that a lot of what we take as "personality" is not fixed at all and instead tends to change with situations.
There are plenty of stories of model citizens going bad. Businessmen trying to prop up their company by all means, including criminal ones, or respected citizens kill their spouses. I think Bernie Madoff was a serious and successful businessman before he started his scam.
I think the personality you see in others depends highly on the situation. Shy people may become extroverted under right circumstances, or stingy people generous.
I would argue that it's easier from someone 'good' to become 'bad', as many of the things people believe that make them good can be incited by novelty, which naturally wears down over time. Kind of like how something can seem amazing the first time you know of it, but after it's been in your head for 20 years it just seems rote.
I'd propose that a complete set of research would learn that there are some personality traits that are malleable and there are some that are not. Moreover, those two sets would not be the same for everyone, and there is likely a great deal of diversity.