It's definitely possible. Or, there's always the possibility that your characters don't want to believe that this person is a dangerous man, and thus the setting you create may feed into that misguided perception because he so aptly portrays what most would consider a normal guy.
Many "normal" guys give something away to show they're not quite there; but, people are quick to shrug it off than take it seriously. You might not know they're cold-blooded killers, but they may say something or act a certain way that could make your blood run cold. Psychopathic serial killers such as Ted Bundy or Paul Bernardo had similar profiles -- clean, good-looking, smart and successful -- no one could believe that either was capable of brutal rape and murder. In Paul's case, a friend was confronted with a hint of his evil ways at a party. A drunk Paul (allegedly) confesses to one of his friends that they don't know the sort of evil that Paul is capable of (paraphrasing).
There are also episodic homicides, meaning that one could assume that a man (or woman) may not be naturally inclined to murder, but it is an event in particular that makes them snap. And because they've never shown anyone traits or qualities that would be of concern, their inexplicable rage and violence appears almost out of nowhere.
In terms of relationships, there have been cases where years after the relationship started, even after marriage and children, the husband (or wife) may become abusive in some way or form. But, it isn't a wake up one day and cause hell scenario. There has to be years of well-hidden emotional turmoil within the person to get to such an episodic point. Or it could take the introduction of a new lover, a loss of a longtime career, or the loss of a loved one that may act as a trigger. Depending on the pov you're writing from, you may have to capture that.
Brain trauma, drugs, and substance abuse also can play a role to sudden violence too.