The answer is complex. Borderlines are some of the most empathetic people, we can feel your pain and will be the first ones to help. The idea of a borderline being mean, likely came from a splitting episode. When we split, often we are in a lot of fear. That fear (of abandonment) immediately puts us into fight or flight and we begin to react in a reflex like display of anger, could even be dangerous.
It's important to know that the person acting out, feels like they are in the backseat watching themselves destroy their lives, but from the backseat, they have no ability to stop it. Frantic efforts are made, then convinced they are being abandoned, the rage begins and can sometimes make those fears a sure reality.
We hate that we do this. These episodes go from intense rage to overwhelming shame that is so bad it causes them to want to die or hurt themselves. They want to punish themselves for how they behaved, and they feel a great deal of regret. This is often when we begin to apologize and attempt to get you to forgive us. We are aware off our BIG reaction, but this behavior is not our personality, and can be easily explained by looking at the brain differences. The brain of someone with BPD is smaller and has reduced activity in 3 areas.
*The amygdala- which plays an important role in regulating emotions. especially the more "negative" emotions, such as fear, aggression and anxiety
*The hippocampus- which helps regulate behavior, impulsivity and self-control
*The orbitofrontal cortex- which is involved in planning and decision making
Problems in these parts of the brain, contribute to the symptoms of BPD, and it is not completely voluntary. The only way to decrease these episodes are intense treatment, like Dialectical Behavior Therapy. That will modify how you perceive things, and provide skills to regulate your emotions. An example might be the skill "check the facts." Being able to stop and ask yourself what is true, and what is just a feeling. This takes a lot of time and hard work, just to appear to respond in a way that comes naturally to you.
So are Borderlines mean? They can be, But it is not because they are hateful, but because the brain abnormalities that can be seen with brain imaging. I hope this knowledge allows you to be more compassionate, and that it removes any judgement. Most people with BPD suffered complex trauma, and the response to that trauma is Borderline Personality Disorder.