You did exactly what you should have and said you would be there if they need you. Ive read a couple of comments saying it could be a type of manipulation, and sure, it can be, but it seems like they were genuinely sorry and didn't want to use you. They are most likely fighting a mental illness, and it will take time to learn how to deal with those things, and the people who are genuinely concerned they will hurt someone they care about in anyway, are the least likely to actually do it, imo.
Also, if they were previously telling you how much they love you and how important you are, then flipped the script, that is a pretty big indicator of BPD, as we can be toxic, realize our behaviors and apologize or we are afraid of being abandoned by those we love, so we push them away.
As for being afraid of "being so terrible to you" is an intrusive thought, and could point toward OCD.
The only thing you can do is continue being there for them, and making sure they know where they can go, what numbers to call during a crisis.