How could someone be so mean and uncaring?

I ended a 20 year relationship to be with someone who professed such love for me. As soon as I ended this relationship they lost interest in about a month. We had been seeing each other, cause I thought my current long term relationship was over. She also was living with someone else. I just don't understand when we were sneaking around must of been more exciting for her, than to really settle down. I haven't spoken to her since, and was very fortuanate to get back into my past relationship. I will never stray again. I just wonder whats up with that kinda thinking, and yes I know it takes two to tangle. I just wonder if this person is mental, or potentially dangerous, like anti-social. And possibly enjoys breaking up marriages, cost people much unneeded expense, and heartache. I really learned my lesson, kinda like fatal attraction.