It happens that a person with undiagnosed and ignored 'borderline personality disorder' be a danger to others?

Not to be flippant,: In a sense - under the "wrong" conditions - anyone can be a danger to others. Until diagnosed, there is no borderline pd, only a difficult, volatile person and compared to an easy going, calm person - yes. If things don't go well for and with them, they can act out in ways that are dangerous. A sad but true thing is, even w/ therapy, many are still not easy to deal with. Don't diagnose. Be safe!