Aren't negative interest rates deflationary by design?
When an entity already in debt borrows money at negative interest rates the debt will keep shrinking, thus destroying monetary supply.
Not an expert either.
I borrow $1000 and promise to pay back $1050. Lender now has a $1050 IOU which he will likely put into circulation (borrow against it, trade it etc.). Monetary supply just increased by $50.