If they are untreated, their emotional development is stuck at about age four, so they are very limited in their ability to protect their fragile egos. They are restricted to using the primitive ego defenses available to a young child. These include denial, projection, double standards, and black-white thinking.
You might have added, throwing and breaking things, one of the key weapons in the four year-old's arsenal. I have never met a woman before who got so violent so quickly over something that merited a cold conversation at best or a shouting match at worse. Her reflex to break, throw and hit was scary to watch, to say the least.
Although most BPDers I've met are very intelligent people, trying to reason with them goes nowhere because it is extremely painful and shameful for them to acknowledge making a mistake. Hence, when you have them in a corner, they usually will not hesitate to try to lie their way out of it. Reasoning does not work because, when they are arguing with you, they "split off" the logical intellectual part of their minds -- putting it out of reach of the conscious part. You therefore are left trying to reason with the emotional, intuitive part of their minds, i.e., with their "inner child."
In fact, I don't think we've ever had a really balanced, enjoyable conversation since we got married. Of course in the almost two years BEFORE we married, this wasn't the case: she was "love-bombing" me, and scored a direct hit every day. I thought I had a soul mate who understood me. Now, apart from not really engaging in my life, she's ever given me a sincere apology for anything since I've known her. Sure, if she spills coffee on me by accident she'd apologize and clean it up, as the obvious social practice. But if she does something wrong that is more subtle or more profound, I get a perfunctory "sorry, it's done", or worse, "what's your problem? what's this long face about?", and not in a caring way.
Yes, as Joe explained, a BPDer has a strong desire to control every aspect of her loved one's life. This is due to her great fear of abandonment, her inability to trust anyone, and her great difficulty with "object constancy," i.e., realizing that other people have stable personalities and desires that are reasonably constant over time. To augment that control, it is common for BPDers to try to isolate their spouses from the other family members and friends who would be supportive.
I just recently realized this dynamic at play here. All of the other women I have had relationships with in the past were wonderful social people. I really liked that, they tended to keep our social life as a couple on track, always planning meetings or dinners or events with family and friends. I didn't care if the events tilted towards her friends, as long as my friends and family were included in our social circle, and respected for their importance to me. Which they always were with these other women.
Now, I notice that my social life has taken a real dive since I met my wife. I find myself making apologies for why we can't accept invitations or can't give a quick answer. And we tend to stay home way too much for a couple without kids and with two incomes.
She has indeed tried to isolate me slowly from friends and family. I only see this now.
My wife is also downright rude and mercurial. An example. About two months ago, an old close business colleague (not a close friend, but a very nice guy with whom I worked closely on a project for two whole years), came to town on a Sunday morning for a business meeting on the Monday. He called to ask if we coud have brunch on the Sunday. My wife said, sure, let's! And, of course, since we're doing some shopping for the house afterwards, if he wants to walk the mall with us, she volunteered that he'd be welcome to come with us. Well, 15 minutes into brunch she decided for some reason she didn't like this guy (no reason I could see), and texted me that she didn't want him to come shopping with us and wanted to finish brunch right away. Could I please ditch him as soon as I can? I was shocked, and didn't ditch my colleague, which really pissed her off. Why would she offer to bring this guy along and then get me to ditch him right away?