Imma call you out on your edit, boy. That shit is in my email. :P

There are two separate things going on with the "fucking shit up." First is the narcissism - people like to have their self-worth confirmed by getting people to like them / fuck them / love them / etc. This is why you often see girls with really low self-esteem being utter whores. Not everyone who has a ton of sex is a whore or has low self esteem, but most girls who have low self esteem run around town...a lot. They're using sex as validation of their self worth.

By taking someone who is already in a relationship, this validation is of a higher caliber than taking someone who is unattached. Not only did you get someone to like / fuck / love you, but you were also chosen over someone else by someone who, as am_Unition said, demonstrates attractiveness and eligibility for further relationships. That's, like, 3x a random single bloke!

The psychopathy comes in a bit differently. There are times when people fuck other people not to confirm their value, but simply because they can. Sometimes people just need sex. Sometimes it's a game. Sometimes people are searching for something different. But, if the person you go after is in an attached relationship and you do purposefully this to fuck up their lives in one way or another, I think that's psychopathy rather than narcissism.

If you do it to purposefully fuck up your own life, that's a whole separate issue that probably stems from fear, boredom, etc. It's kinda hard being really happy with your life and circumstances. We all want more, all the time. But if everything is perfect, what more can you want? What challenges can you conquer to learn / grow / explore / obtain excitement? Obviously, there are a ton of alternatives to fucking a random girl in a bar, losing your wife, losing your house, but people still do it. Then they have an entirely new set of challenges to overcome. No more boredom. No more being unsatisfied with being satisfied.

Also, people sometimes just need some good, NSA sex. Sometimes this may help them to realize the relationship they were in wasn't working out. The sex / traveling across the county on a whim simply to fuck for a weekend was the vessel that made them reexamine their circumstances. "Why did I do that?" turns into "what am I doing with my life / with my partner?" turns into "I want more / I'm unsatisfied / damn, that was some good sex, can I have more of that?"

Eventually, those people would have figured out and gotten out of the relationship for any number of reasons. But when you fuck someone else, it fast tracks the process.