I have BPD, and due to that my behavior is somewhat unpredictable even to me. I often end up doing things that are outright cruel even to my friends and family. My question is, am I a bad person? Even if I have a real issue am I not still responsible for my actions and how they impact others?
Let's break this question down into a couple of different areas.
Legally, in most countries you have to be of sound mind to stand trial for serious crimes. In many of those cases, intent is a serious consideration of whether the crime belongs in a specific category.
For instance, murder may be considered murder only if there was pre-meditation (planning and lingering intent). If you on the other hand get into a fit of rage and do something really stupid that causes another to end up being injured fatally, that's probably manslaughter.
This is not a free pass if you're found NOT to be of sound mind; in such a case you may well be remanded into a psychiatric facility if you commit what is a serious crime (or a crime with serious consequences) but you're found unfit to stand trial.
Morally, one could argue that the boundary of personal responsibility is tied to the state of one's mental disorder. If you do things that are genuinely outside your control, then you are unlikely to be responsible for them. Despite this, it is important to note that pain is still incurred by your actions to others and the lack of 'responsibility' does not extend to a lack of consequence. Ultimately, whether you possessed the power to control your actions or not doesn't negate the impact you have on others. As such, you may still bear a responsibility to address those impacts, through apologising or making amends in some form. BPD or any other mental disorder may render you incapable of control but that increases your personal responsibility to isolate yourself where possible if you feel like you may lose control of your actions and hurt others, be it emotionally or physically.
That said, there is an important difference here between immorality and amorality. An immoral person knows the difference between right and wrong and chooses to act in a wrong way where convenient. In such a case, addressing the wrong behaviour is a better solution than addressing the wrong 'person', so to speak. In your case, the fact that you question whether or not this behaviour outside your control means that when you are not in the throes of a bipolar episode, you have a moral understanding and that understanding means that you should treat the consequences of those outbursts as immoral acts and make appropriate remedy to others.
While you're in the throes of the episode and unable to control it however, you are acting in an amoral manner. Amoral people don't have a concept of right or wrong; only what is expedient and what achieves their goals in the most efficient manner. Often such people are still willing to follow the rules of society as the sanctions imposed for not doing so are inconvenient. In other words, they do good because of the penalty for not doing so, not because of the merit in doing so. This is a behaviour model we often describe as psychopathic.
With BPD, you can't change that at the time, but you can address it afterwards. If you do so in accordance with your own moral focus, then you are not a 'bad' person; you are a person who sometimes does bad things because of a medical condition. If on the other hand you say to yourself 'Well, I wasn't in control so I have no need to apologise or make amends', then you know the difference between good and bad, but you're choosing to accept the bad you've done along the way.
Whether that makes you good or bad is up to you, but it certainly reflects an immoral character's behaviour. It is most important in these discussions to separate responsibility for one's actions from the consequences of one's actions. By reflecting on the latter, the former can be far better understood in my experience.