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I feel you, my man. It's actually not uncommon to go on "offense" after a relationship like this. Psychologically, what happens is that you want to regain some control over the situation after being manipulated and emotionally abused for X amount of time. It can do a number on someone's subconscious. If you want a mainstream example, look at how Calvin Harris and Kanye West promptly put Taylor Swift in her place over the past few months - they weren't in the mood for someone to walk all over them.
So I can't say you did anything out of the ordinary, in fact I'd say it's probably healthy you were quite proactive after the split. It's much better than continuing to be a lapdog and subjecting yourself to months, if not years of unique emotional turmoil.
As for the guilt, this is something I too struggle with.We're all here for the same reason, so my story doesn't differ too much from yours. I too for a number of years was involved with a girl suffering from BPD - starting right as I graduated high school, and continuing into my 20's. To prevent a mammoth wall of text from being unleashed, the simplistic version is that we got to a point where her disorder destroyed the relationship.
Like you, I went on offense. I introduced myself to the online girlfriend she'd been bragging about, which ended that relationship. I threw up a piece about her on TheDirty, but not before strategically editing the article to ensure it was nothing but the truth - ensuring that she couldn't sue me for libel nor get the post removed. I went to her younger sister in such a way that she actually began opening up to me about my ex's problems, essentially turning her God-given best friend against her. And lastly, when she began stalking me in the ways only a hot, crazy girl can muster, I didn't brush it aside; first the police were involved, and then civil litigation. She was fired from her job, her parents exposed to the full extent of her absurd behavior thanks to detailed legal documents they most likely went over as a family, and of course with the way their emotions work, a simple visit from the police is enough to send them spiraling out of control.
Now you might actually read this and believe I did the right thing - and this is quite similar to how I read your post and believe YOU did the right thing.
However, I feel extreme guilt over my actions on a daily basis. The last time I ran her name through Google, I discovered she's homeless in Australia. By comparison, I'm an amateur race car driver. While she is on various forms of social media begging for a couch to sleep on, I get to live out the dreams of every six year old. And I'm fully aware that I helped contribute to her turmoil. Ruined a relationship she valued with an online friend, publicly humiliated her, turned her sister against her, got her in trouble with the police, exposed her in front of her parents, and got her fired from a job that actually suited her fairly well. For as much as her disorder contributed to the specific details of these events, let's be real - I played a role in this.
It's something I think about every day. The most logical reason is that we have a sort of self-awareness and general compassion for other human beings who are suffering. I mean, there are days where I'd kill just to have one final conversation in some dingy coffee shop just to try and understand the mess a little better.
But then you look at old messages and you're like "oh God nevermind, this girl was insane, that scenario would last about 30 seconds before objects would be flying at my head."
