Without jumping into the inflation/deflation debate (that's where this thread is headed, I promise), because all that will result in is me wanting to bang my head against a wall, negative interest rates are an inflationary policy. They're trying to avoid what they think could turn into a deflationary cycle.
To make it simple:
The lower the interest rate, the cheaper it is to borrow money, the more inflationary the policy.
The higher the interest rate, the more expensive is it to borrow money, the more deflationary the policy.