How can people with virtues fall prey to human instincts?

First of all, we must note that we cannot prove that any answer to this question is objectively true.  We cannot do experiments to find out what would cause “people with virtues” to lose those virtues and “fall prey to human instincts.”  In addition, we would have a very hard time identifying which people are “people with virtues” and we would have a very hard time defining what it means to “fall prey to human instincts.”  For example, let us say that a man has never been violent in his life.  Does that mean he has virtues or does it mean he has not been put in situations where he needed to be violent?  Then let us say he does something violent for the first time.  In what circumstances will we say he has “fallen prey to human instincts?”  If he is violent to protect someone else, is that human instinct or virtue?  In short, it is not possible to answer this question in a scientific way.

When we do answer this question, we also have to realize that people might disagree on the answer.  A religious person, for example, might say that the person has been tempted by Satan.  Meanwhile, a non-religious person, or a person from a different religious tradition, might disagree.  My own view is that our human nature causes every person to act, at least sometimes, according to instinct and not to virtue.

It seems clear to me that our human nature prevents us from being perfectly virtuous.  We human beings feel urges at almost all times that are not virtuous.  We feel greed, anger, lust, and other things that most people say are not virtuous.  In order to always be virtuous, we would have to suppress those feelings successfully 100% of the time.  This is surely impossible.  Even if we normally suppress our instincts almost all the time, there will be times when we do not. 

When will those times be?  I would argue that they will be A) when temptation is greatest and/or B) when our ability to control ourselves is low.  So, when might a person “fall prey to human instincts?”  They might do this when there is a great temptation.  Let us say you are a fundamentally honest person but you are suddenly presented with a chance to take $1 million that does not belong to you without much risk of being caught.  Let us say that you are usually not violent, but someone has said terrible things to you and spit on your shoes.  At times like these, the temptation to give in to our instincts might overwhelm our ability to suppress them.  Humans might also give in to temptation when they are at a weak point in their lives.  Let us say that we are under great stress because of our job or our family life.  Let us say that we are very tired.  At times like these, we might lack the strength to resist temptation and we might give in to our human instincts.

My own view, then, is that a virtuous person can fall prey to their human instincts if the temptation is strong enough and/or if they are in a situation where they are not as strong as usual.  However, this is not an answer that can be proven scientifically and other people might disagree with it.