Opening a short position on Kraken requires BTC to begin with?
You have to get money into the exchange to purchase and secure your position somehow. BTC is probably an easy way to do it but you could also do it with EUR or USD. You will need BTC to close the position but since you're going to be shorting BTC, you won't want to hold BTC while the position is open, so you'll be holding some form of fiat instead.