Response/ comments in line:
I don't know where it filled up, but apt-get install filelight to see where the space is. (also link there shows more info).The ONLY thing I ever did was run a few airmon-ng commands and view some html files in Firefox. Why and where did Linux fill up all remaining disk space and how do I get it back?
Yes you can delete anything you want. But you may in turn mess something up.Can I delete some major package to free up 100MB or so? I only use the airmon-package.
Send all output to /dev/null but that's probably not the best solution. A better solution to this entire issue would be to give BT more space.And how can I make sure Linux doesn't fill up all remaining space again (that's worse than Windows!).
Further let's be honest here, Windows in all of it's recent and or supported variants require a lot more space than 5, gigs so if you don't like BT feel free to use something else.