I don't understand negative interest rates at all, but surely something is deeply broken behind this, and it will crash in one spectacular way or another.
It's a cost of having money, but not spending it. The central bank pays a negative interest on excess deposits. So banks are motivated to lend it out. (Eg. take on more risk.)
One problem that becomes a bit hard is that in the retail sector if interest rates go below zero people are willing to simply withdraw the money, which would hurt banks' ability to lend. (Eg. the central bank would have to add funding via some mechanism, such as lowering the fractional ratio, or QE . [Or paying interest on reserves. But that would go against the negative interest on excess reserves.])
The paper concludes that the negative interest rates resulted in more loans.
Money is just a piece of the underlying monetary system, which is constantly priced based on what the economy using that money does.
If that economy expands without corresponding increase in the money supply, then prices go down (because there are more stuff, but the same amount of money, so the same amount of money now represent more stuff). But this represents a deflation, which would auto-magically counteract the expansion, because it would incentivize people to wait and spend just at the last minute. (Time value of money and all would revert, wages would fall, fixed amount mortgages would start to spiral out, and all the regular deflation doomsday scenarios.) So the central banks make sure that even in an expanding economy the money supply "stays ahead" of the expansion of the economy. Targeting 2%. (And usually falling short, so inflation is somewhere between 1-2%.)
So holding money did not became worthless, quite the opposite, holding cash is now better than holding it in a bank. But the central bank has only a few policy tools available, and one of them is the interest rate manipulation.
The central banks want to encourage people to make some investments, to take on some risk, or at least spend. Sure, one way is to simply fund the government, as that usually seems the least risky. And it's not like there's nothing to spend on - the Green New Deal and co, but governments are quite reluctant to do so.
No, what's actually broken is positive investment rates. Why would the central bank pay you (i.e. print new money and give it to you) just because you have some money lying on the central bank's account? And by "you" of course I mean big 1% banks, not the 99% population who don't have access to this feature. Commercial banks should be making money by taking consumers' deposits and loaning them out to businesses, taking a cut of the earned interest. If they can't do that, do they actually have a reason to exist?
