Can you just “snap” one day?

I have been extremely interested in the Chris Watts case and have been following it closely. If you don’t know the case, please google it. Everyone interviewed, except for maybe one or two people, have said what a nice, quiet, family centered guy this man was. In the confession, Watts himself told investigators it was like someone else was holding his hands around his wife’s neck the day he strangled her and killed his two kids. Watts said the murders weren’t premeditated, but that he snapped after he told his wife he no longer loved her and she threatened to take the kids.

However, days prior to the murders he called a realtor and asked about selling the couple’s (unbeknownst to his wife) house, and called the kids’ school and said they’d no longer be attending. The Watts’ were having marital issues (he was having an affair), were in debt, and there was a third child on the way.

In short, do you think it’s possible for someone who seems so normal to everyone around them to “snap” one day, due to family stressors, and kill their spouse/family? Or, do you think that person has always had those homicidal tendencies and has just managed to hide or suppress it?

Edited to add: Do you think he would have murdered his family if he’d had an affair, or kept up the facade?