I'm not a mental health professional either despite what my facebook says.
Disordered people, just like regular folks, have stressors and triggers too in my experience. My ex was diagnosed as comborbid APD/NPD (and I believe he would have been dx'd with conduct disorder but he was in counseling before that was even a dx). Prior to my knowing him he'd had some minor scrapes with the law but had been lucky and avoided incarceration. He had a further history of arson as a young adult I only knew about after he set our house on fire when we'd been together many years.
Prior to his diagnosis though he was married twice, once for 7 years and once for 10 years. He had three children and maintained relatively continuous employment. He had several friends on a very superficial level and some semblance of a relationship with his family. While he was abusive during our entire relationship it wasn't until the birth of our second child and financial stressors came about that his 'condition' began worsening. Fast forward a few years later, after the death of my parents, and he was arrested, plead guilty to, and convicted of drugging and raping his 14 year old niece. He was 43 years old at the time. For many years though he appeared 'normal' to those who did not know him intimately. Myself, and many, many others excused and minimized his behavior for much of it. (He was just stressed; he just had a temper; I'd just been thoughtless, etc.)
I believe disordereds are fully capable of maintaining facades...the effectiveness and length of such are determined in part by those surrounding them as much as everyday stressors. JMO